author | kevin@6e1638ff-ae45-0410-89bd-df963105f760 |
Wed, 27 Jan 2010 18:33:59 +0000 | |
changeset 199 | a2ff2d278b97 |
parent 198 | 1eab7b40e897 |
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permissions | -rw-r--r-- |
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%!TEX root = ../blob1.tex |
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\def\xxpar#1#2{\smallskip\noindent{\bf #1} {\it #2} \smallskip} |
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\def\mmpar#1#2#3{\smallskip\noindent{\bf #1} (#2). {\it #3} \smallskip} |
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\section{$n$-categories} |
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\label{sec:ncats} |
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\subsection{Definition of $n$-categories} |
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Before proceeding, we need more appropriate definitions of $n$-categories, |
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$A_\infty$ $n$-categories, modules for these, and tensor products of these modules. |
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(As is the case throughout this paper, by ``$n$-category" we implicitly intend some notion of |
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a `weak' $n$-category with `strong duality'.) |
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The definitions presented below tie the categories more closely to the topology |
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and avoid combinatorial questions about, for example, the minimal sufficient |
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collections of generalized associativity axioms; we prefer maximal sets of axioms to minimal sets. |
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For examples of topological origin, it is typically easy to show that they |
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satisfy our axioms. |
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For examples of a more purely algebraic origin, one would typically need the combinatorial |
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results that we have avoided here. |
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\medskip |
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Consider first ordinary $n$-categories. |
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\nn{Actually, we're doing both plain and infinity cases here} |
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We need a set (or sets) of $k$-morphisms for each $0\le k \le n$. |
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We must decide on the ``shape" of the $k$-morphisms. |
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Some $n$-category definitions model $k$-morphisms on the standard bihedron (interval, bigon, ...). |
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Other definitions have a separate set of 1-morphisms for each interval $[0,l] \sub \r$, |
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a separate set of 2-morphisms for each rectangle $[0,l_1]\times [0,l_2] \sub \r^2$, |
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and so on. |
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(This allows for strict associativity.) |
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Still other definitions \nn{need refs for all these; maybe the Leinster book \cite{MR2094071}} |
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model the $k$-morphisms on more complicated combinatorial polyhedra. |
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We will allow our $k$-morphisms to have any shape, so long as it is homeomorphic to |
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the standard $k$-ball. |
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In other words, |
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\begin{preliminary-axiom}{\ref{axiom:morphisms}}{Morphisms} |
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For any $k$-manifold $X$ homeomorphic |
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to the standard $k$-ball, we have a set of $k$-morphisms |
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$\cC_k(X)$. |
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\end{preliminary-axiom} |
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Terminology: By ``a $k$-ball" we mean any $k$-manifold which is homeomorphic to the |
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standard $k$-ball. |
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We {\it do not} assume that it is equipped with a |
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preferred homeomorphism to the standard $k$-ball. |
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The same goes for ``a $k$-sphere" below. |
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Given a homeomorphism $f:X\to Y$ between $k$-balls (not necessarily fixed on |
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the boundary), we want a corresponding |
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bijection of sets $f:\cC(X)\to \cC(Y)$. |
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(This will imply ``strong duality", among other things.) |
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So we replace the above with |
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\begin{axiom}[Morphisms] |
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\label{axiom:morphisms} |
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For each $0 \le k \le n$, we have a functor $\cC_k$ from |
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the category of $k$-balls and |
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homeomorphisms to the category of sets and bijections. |
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\end{axiom} |
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(Note: We usually omit the subscript $k$.) |
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We are so far being deliberately vague about what flavor of $k$-balls |
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we are considering. |
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They could be unoriented or oriented or Spin or $\mbox{Pin}_\pm$. |
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They could be topological or PL or smooth. |
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%\nn{need to check whether this makes much difference} |
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(If smooth, ``homeomorphism" should be read ``diffeomorphism", and we would need |
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to be fussier about corners.) |
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For each flavor of manifold there is a corresponding flavor of $n$-category. |
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We will concentrate on the case of PL unoriented manifolds. |
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Next we consider domains and ranges of morphisms (or, as we prefer to say, boundaries |
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of morphisms). |
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The 0-sphere is unusual among spheres in that it is disconnected. |
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Correspondingly, for 1-morphisms it makes sense to distinguish between domain and range. |
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(Actually, this is only true in the oriented case, with 1-morphsims parameterized |
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by oriented 1-balls.) |
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For $k>1$ and in the presence of strong duality the domain/range division makes less sense. |
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\nn{maybe say more here; rotate disk, Frobenius reciprocity blah blah} |
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We prefer to combine the domain and range into a single entity which we call the |
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boundary of a morphism. |
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Morphisms are modeled on balls, so their boundaries are modeled on spheres: |
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\begin{axiom}[Boundaries (spheres)] |
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For each $0 \le k \le n-1$, we have a functor $\cC_k$ from |
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the category of $k$-spheres and |
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homeomorphisms to the category of sets and bijections. |
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\end{axiom} |
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(In order to conserve symbols, we use the same symbol $\cC_k$ for both morphisms and boundaries.) |
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\begin{axiom}[Boundaries (maps)]\label{nca-boundary} |
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For each $k$-ball $X$, we have a map of sets $\bd: \cC(X)\to \cC(\bd X)$. |
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These maps, for various $X$, comprise a natural transformation of functors. |
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\end{axiom} |
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(Note that the first ``$\bd$" above is part of the data for the category, |
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while the second is the ordinary boundary of manifolds.) |
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Given $c\in\cC(\bd(X))$, let $\cC(X; c) \deq \bd^{-1}(c)$. |
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Most of the examples of $n$-categories we are interested in are enriched in the following sense. |
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The various sets of $n$-morphisms $\cC(X; c)$, for all $n$-balls $X$ and |
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all $c\in \cC(\bd X)$, have the structure of an object in some auxiliary category |
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(e.g.\ vector spaces, or modules over some ring, or chain complexes), |
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and all the structure maps of the $n$-category should be compatible with the auxiliary |
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category structure. |
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Note that this auxiliary structure is only in dimension $n$; |
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$\cC(Y; c)$ is just a plain set if $\dim(Y) < n$. |
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\medskip |
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\nn{ |
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%At the moment I'm a little confused about orientations, and more specifically |
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%about the role of orientation-reversing maps of boundaries when gluing oriented manifolds. |
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Maybe need a discussion about what the boundary of a manifold with a |
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structure (e.g. orientation) means. |
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Tentatively, I think we need to redefine the oriented boundary of an oriented $n$-manifold. |
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Instead of an ordinary oriented $(n-1)$-manifold via the inward (or outward) normal |
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first (or last) convention, perhaps it is better to define the boundary to be an $(n-1)$-manifold |
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equipped with an orientation of its once-stabilized tangent bundle. |
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Similarly, in dimension $n-k$ we would have manifolds equipped with an orientation of |
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their $k$ times stabilized tangent bundles. |
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(cf. [Stolz and Teichner].) |
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Probably should also have a framing of the stabilized dimensions in order to indicate which |
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side the bounded manifold is on. |
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For the moment just stick with unoriented manifolds.} |
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\medskip |
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We have just argued that the boundary of a morphism has no preferred splitting into |
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domain and range, but the converse meets with our approval. |
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That is, given compatible domain and range, we should be able to combine them into |
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the full boundary of a morphism: |
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\begin{axiom}[Boundary from domain and range] |
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Let $S = B_1 \cup_E B_2$, where $S$ is a $k$-sphere $(0\le k\le n-1)$, |
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$B_i$ is a $k$-ball, and $E = B_1\cap B_2$ is a $k{-}1$-sphere (Figure \ref{blah3}). |
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Let $\cC(B_1) \times_{\cC(E)} \cC(B_2)$ denote the fibered product of the |
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two maps $\bd: \cC(B_i)\to \cC(E)$. |
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Then we have an injective map |
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\[ |
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\gl_E : \cC(B_1) \times_{\cC(E)} \cC(B_2) \to \cC(S) |
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\] |
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which is natural with respect to the actions of homeomorphisms. |
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\end{axiom} |
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\begin{figure}[!ht] |
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$$ |
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\begin{tikzpicture}[every label/.style={green}] |
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\node[fill=black, circle, label=below:$E$](S) at (0,0) {}; |
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\node[fill=black, circle, label=above:$E$](N) at (0,2) {}; |
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\draw (S) arc (-90:90:1); |
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\draw (N) arc (90:270:1); |
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\node[left] at (-1,1) {$B_1$}; |
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\node[right] at (1,1) {$B_2$}; |
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\end{tikzpicture} |
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$$ |
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$$\mathfig{.4}{tempkw/blah3}$$ |
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\caption{Combining two balls to get a full boundary |
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\nn{maybe smaller dots for $E$ in pdf fig}}\label{blah3}\end{figure} |
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Note that we insist on injectivity above. |
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Let $\cC(S)_E$ denote the image of $\gl_E$. |
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We will refer to elements of $\cC(S)_E$ as ``splittable along $E$" or ``transverse to $E$". |
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If $X$ is a $k$-ball and $E \sub \bd X$ splits $\bd X$ into two $k{-}1$-balls $B_1$ and $B_2$ |
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as above, then we define $\cC(X)_E = \bd^{-1}(\cC(\bd X)_E)$. |
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We will call the projection $\cC(S)_E \to \cC(B_i)$ |
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a {\it restriction} map and write $\res_{B_i}(a)$ |
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(or simply $\res(a)$ when there is no ambiguity), for $a\in \cC(S)_E$. |
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More generally, we also include under the rubric ``restriction map" the |
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the boundary maps of Axiom \ref{nca-boundary} above, |
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another calss of maps introduced after Axion \ref{nca-assoc} below, as well as any composition |
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of restriction maps (inductive definition). |
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In particular, we have restriction maps $\cC(X)_E \to \cC(B_i)$ |
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($i = 1, 2$, notation from previous paragraph). |
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These restriction maps can be thought of as |
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domain and range maps, relative to the choice of splitting $\bd X = B_1 \cup_E B_2$. |
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Next we consider composition of morphisms. |
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For $n$-categories which lack strong duality, one usually considers |
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$k$ different types of composition of $k$-morphisms, each associated to a different direction. |
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(For example, vertical and horizontal composition of 2-morphisms.) |
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In the presence of strong duality, these $k$ distinct compositions are subsumed into |
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one general type of composition which can be in any ``direction". |
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\begin{axiom}[Composition] |
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Let $B = B_1 \cup_Y B_2$, where $B$, $B_1$ and $B_2$ are $k$-balls ($0\le k\le n$) |
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and $Y = B_1\cap B_2$ is a $k{-}1$-ball (Figure \ref{blah5}). |
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Let $E = \bd Y$, which is a $k{-}2$-sphere. |
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Note that each of $B$, $B_1$ and $B_2$ has its boundary split into two $k{-}1$-balls by $E$. |
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We have restriction (domain or range) maps $\cC(B_i)_E \to \cC(Y)$. |
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Let $\cC(B_1)_E \times_{\cC(Y)} \cC(B_2)_E$ denote the fibered product of these two maps. |
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Then (axiom) we have a map |
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\[ |
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\gl_Y : \cC(B_1)_E \times_{\cC(Y)} \cC(B_2)_E \to \cC(B)_E |
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\] |
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which is natural with respect to the actions of homeomorphisms, and also compatible with restrictions |
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to the intersection of the boundaries of $B$ and $B_i$. |
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If $k < n$ we require that $\gl_Y$ is injective. |
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(For $k=n$, see below.) |
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\end{axiom} |
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\begin{figure}[!ht] |
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$$\mathfig{.4}{tempkw/blah5}$$ |
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\caption{From two balls to one ball}\label{blah5}\end{figure} |
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\begin{axiom}[Strict associativity] \label{nca-assoc} |
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The composition (gluing) maps above are strictly associative. |
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\end{axiom} |
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\begin{figure}[!ht] |
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$$\mathfig{.65}{tempkw/blah6}$$ |
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\caption{An example of strict associativity}\label{blah6}\end{figure} |
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\nn{figure \ref{blah6} (blah6) needs a dotted line in the south split ball} |
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Notation: $a\bullet b \deq \gl_Y(a, b)$ and/or $a\cup b \deq \gl_Y(a, b)$. |
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In the other direction, we will call the projection from $\cC(B)_E$ to $\cC(B_i)_E$ |
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a restriction map (one of many types of map so called) and write $\res_{B_i}(a)$ for $a\in \cC(B)_E$. |
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%Compositions of boundary and restriction maps will also be called restriction maps. |
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%For example, if $B$ is a $k$-ball and $Y\sub \bd B$ is a $k{-}1$-ball, there is a |
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%restriction map from $\cC(B)_{\bd Y}$ to $\cC(Y)$. |
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We will write $\cC(B)_Y$ for the image of $\gl_Y$ in $\cC(B)$. |
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We will call $\cC(B)_Y$ morphisms which are splittable along $Y$ or transverse to $Y$. |
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We have $\cC(B)_Y \sub \cC(B)_E \sub \cC(B)$. |
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More generally, let $\alpha$ be a subdivision of a ball (or sphere) $X$ into smaller balls. |
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Let $\cC(X)_\alpha \sub \cC(X)$ denote the image of the iterated gluing maps from |
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the smaller balls to $X$. |
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We will also say that $\cC(X)_\alpha$ are morphisms which are splittable along $\alpha$. |
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In situations where the subdivision is notationally anonymous, we will write |
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$\cC(X)\spl$ for the morphisms which are splittable along (a.k.a.\ transverse to) |
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the unnamed subdivision. |
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If $\beta$ is a subdivision of $\bd X$, we define $\cC(X)_\beta \deq \bd\inv(\cC(\bd X)_\beta)$; |
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this can also be denoted $\cC(X)\spl$ if the context contains an anonymous |
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subdivision of $\bd X$ and no competing subdivision of $X$. |
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The above two composition axioms are equivalent to the following one, |
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which we state in slightly vague form. |
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\xxpar{Multi-composition:} |
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{Given any decomposition $B = B_1\cup\cdots\cup B_m$ of a $k$-ball |
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into small $k$-balls, there is a |
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map from an appropriate subset (like a fibered product) |
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of $\cC(B_1)\spl\times\cdots\times\cC(B_m)\spl$ to $\cC(B)\spl$, |
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and these various $m$-fold composition maps satisfy an |
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operad-type strict associativity condition (Figure \ref{blah7}).} |
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\begin{figure}[!ht] |
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$$\mathfig{.8}{tempkw/blah7}$$ |
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\caption{Operadish composition and associativity}\label{blah7}\end{figure} |
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The next axiom is related to identity morphisms, though that might not be immediately obvious. |
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\begin{axiom}[Product (identity) morphisms] |
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For each $k$-ball $X$ and $m$-ball $D$, with $k+m \le n$, there is a map $\cC(X)\to \cC(X\times D)$, usually denoted $a\mapsto a\times D$ for $a\in \cC(X)$. These maps must satisfy the following conditions. |
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\begin{enumerate} |
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\item |
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If $f:X\to X'$ and $\tilde{f}:X\times D \to X'\times D'$ are maps such that the diagram |
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\[ \xymatrix{ |
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X\times D \ar[r]^{\tilde{f}} \ar[d]_{\pi} & X'\times D' \ar[d]^{\pi} \\ |
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X \ar[r]^{f} & X' |
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} \] |
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commutes, then we have |
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\[ |
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\tilde{f}(a\times D) = f(a)\times D' . |
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\] |
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\item |
109 | 282 |
Product morphisms are compatible with gluing (composition) in both factors: |
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\[ |
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(a'\times D)\bullet(a''\times D) = (a'\bullet a'')\times D |
|
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\] |
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and |
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\[ |
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(a\times D')\bullet(a\times D'') = a\times (D'\bullet D'') . |
|
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\] |
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\nn{if pinched boundary, then remove first case above} |
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\item |
109 | 292 |
Product morphisms are associative: |
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\[ |
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(a\times D)\times D' = a\times (D\times D') . |
|
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\] |
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(Here we are implicitly using functoriality and the obvious homeomorphism |
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$(X\times D)\times D' \to X\times(D\times D')$.) |
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\item |
110 | 299 |
Product morphisms are compatible with restriction: |
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\[ |
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\res_{X\times E}(a\times D) = a\times E |
|
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\] |
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for $E\sub \bd D$ and $a\in \cC(X)$. |
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\end{enumerate} |
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\end{axiom} |
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\nn{need even more subaxioms for product morphisms?} |
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\nn{Almost certainly we need a little more than the above axiom. |
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More specifically, in order to bootstrap our way from the top dimension |
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properties of identity morphisms to low dimensions, we need regular products, |
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pinched products and even half-pinched products. |
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I'm not sure what the best way to cleanly axiomatize the properties of these various |
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products is. |
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For the moment, I'll assume that all flavors of the product are at |
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our disposal, and I'll plan on revising the axioms later.} |
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||
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\nn{current idea for fixing this: make the above axiom a ``preliminary version" |
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(as we have already done with some of the other axioms), then state the official |
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axiom for maps $\pi: E \to X$ which are almost fiber bundles. |
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one option is to restrict E to be a (full/half/not)-pinched product (up to homeo). |
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the alternative is to give some sort of local criterion for what's allowed. |
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state a gluing axiom for decomps $E = E'\cup E''$ where all three are of the correct type. |
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} |
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All of the axioms listed above hold for both ordinary $n$-categories and $A_\infty$ $n$-categories. |
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The last axiom (below), concerning actions of |
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homeomorphisms in the top dimension $n$, distinguishes the two cases. |
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||
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We start with the plain $n$-category case. |
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\begin{preliminary-axiom}{\ref{axiom:extended-isotopies}}{Isotopy invariance in dimension $n$} |
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Let $X$ be an $n$-ball and $f: X\to X$ be a homeomorphism which restricts |
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to the identity on $\bd X$ and is isotopic (rel boundary) to the identity. |
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Then $f$ acts trivially on $\cC(X)$; $f(a) = a$ for all $a\in \cC(X)$. |
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\end{preliminary-axiom} |
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This axiom needs to be strengthened to force product morphisms to act as the identity. |
103 | 339 |
Let $X$ be an $n$-ball and $Y\sub\bd X$ be an $n{-}1$-ball. |
96 | 340 |
Let $J$ be a 1-ball (interval). |
341 |
We have a collaring homeomorphism $s_{Y,J}: X\cup_Y (Y\times J) \to X$. |
|
122 | 342 |
(Here we use the ``pinched" version of $Y\times J$. |
343 |
\nn{need notation for this}) |
|
96 | 344 |
We define a map |
345 |
\begin{eqnarray*} |
|
346 |
\psi_{Y,J}: \cC(X) &\to& \cC(X) \\ |
|
347 |
a & \mapsto & s_{Y,J}(a \cup ((a|_Y)\times J)) . |
|
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\end{eqnarray*} |
|
142 | 349 |
(See Figure \ref{glue-collar}.) |
189 | 350 |
\begin{figure}[!ht] |
351 |
\begin{equation*} |
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\begin{tikzpicture} |
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\def\rad{1} |
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\def\srad{0.75} |
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\def\gap{4.5} |
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\foreach \i in {0, 1, 2} { |
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\node(\i) at ($\i*(\gap,0)$) [draw, circle through = {($\i*(\gap,0)+(\rad,0)$)}] {}; |
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\node(\i-small) at (\i.east) [circle through={($(\i.east)+(\srad,0)$)}] {}; |
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\foreach \n in {1,2} { |
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\fill (intersection \n of \i-small and \i) node(\i-intersection-\n) {} circle (2pt); |
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} |
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} |
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\begin{scope}[decoration={brace,amplitude=10,aspect=0.5}] |
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\end{scope} |
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\node[right=1mm] at (0.east) {$a$}; |
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} |
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} |
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\node[below] at (1-small.south) {$a \times J$}; |
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\begin{scope} |
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\path[clip] (2) circle (\rad); |
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\draw[clip] (2.east) circle (\srad); |
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\foreach \y in {1, 0.86, ..., -1} { |
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\draw[blue] ($(2)+(-1,\y) $)-- ($(2)+(1,\y)$); |
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} |
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\end{scope} |
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\end{tikzpicture} |
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\end{equation*} |
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\begin{equation*} |
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391 |
\xymatrix@C+2cm{\cC(X) \ar[r]^(0.45){\text{glue}} & \cC(X \cup \text{collar}) \ar[r]^(0.55){\text{homeo}} & \cC(X)} |
189 | 392 |
\end{equation*} |
393 |
||
394 |
\caption{Extended homeomorphism.}\label{glue-collar}\end{figure} |
|
174 | 395 |
We say that $\psi_{Y,J}$ is {\it extended isotopic} to the identity map. |
396 |
\nn{bad terminology; fix it later} |
|
397 |
\nn{also need to make clear that plain old isotopic to the identity implies |
|
398 |
extended isotopic} |
|
97 | 399 |
\nn{maybe remark that in some examples (e.g.\ ones based on sub cell complexes) |
400 |
extended isotopies are also plain isotopies, so |
|
401 |
no extension necessary} |
|
96 | 402 |
It can be thought of as the action of the inverse of |
403 |
a map which projects a collar neighborhood of $Y$ onto $Y$. |
|
404 |
||
405 |
The revised axiom is |
|
406 |
||
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407 |
\stepcounter{axiom} |
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\begin{axiom-numbered}{\arabic{axiom}a}{Extended isotopy invariance in dimension $n$} |
187 | 409 |
\label{axiom:extended-isotopies} |
410 |
Let $X$ be an $n$-ball and $f: X\to X$ be a homeomorphism which restricts |
|
174 | 411 |
to the identity on $\bd X$ and is extended isotopic (rel boundary) to the identity. |
187 | 412 |
Then $f$ acts trivially on $\cC(X)$. |
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413 |
\end{axiom-numbered} |
96 | 414 |
|
415 |
\nn{need to rephrase this, since extended isotopies don't correspond to homeomorphisms.} |
|
94 | 416 |
|
97 | 417 |
\smallskip |
418 |
||
419 |
For $A_\infty$ $n$-categories, we replace |
|
420 |
isotopy invariance with the requirement that families of homeomorphisms act. |
|
421 |
For the moment, assume that our $n$-morphisms are enriched over chain complexes. |
|
422 |
||
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423 |
\begin{axiom-numbered}{\arabic{axiom}b}{Families of homeomorphisms act in dimension $n$} |
187 | 424 |
For each $n$-ball $X$ and each $c\in \cC(\bd X)$ we have a map of chain complexes |
97 | 425 |
\[ |
426 |
C_*(\Homeo_\bd(X))\ot \cC(X; c) \to \cC(X; c) . |
|
427 |
\] |
|
428 |
Here $C_*$ means singular chains and $\Homeo_\bd(X)$ is the space of homeomorphisms of $X$ |
|
429 |
which fix $\bd X$. |
|
430 |
These action maps are required to be associative up to homotopy |
|
431 |
\nn{iterated homotopy?}, and also compatible with composition (gluing) in the sense that |
|
432 |
a diagram like the one in Proposition \ref{CDprop} commutes. |
|
433 |
\nn{repeat diagram here?} |
|
187 | 434 |
\nn{restate this with $\Homeo(X\to X')$? what about boundary fixing property?} |
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435 |
\end{axiom-numbered} |
97 | 436 |
|
437 |
We should strengthen the above axiom to apply to families of extended homeomorphisms. |
|
109 | 438 |
To do this we need to explain how extended homeomorphisms form a topological space. |
97 | 439 |
Roughly, the set of $n{-}1$-balls in the boundary of an $n$-ball has a natural topology, |
440 |
and we can replace the class of all intervals $J$ with intervals contained in $\r$. |
|
441 |
\nn{need to also say something about collaring homeomorphisms.} |
|
442 |
\nn{this paragraph needs work.} |
|
443 |
||
103 | 444 |
Note that if we take homology of chain complexes, we turn an $A_\infty$ $n$-category |
445 |
into a plain $n$-category (enriched over graded groups). |
|
97 | 446 |
\nn{say more here?} |
447 |
In the other direction, if we enrich over topological spaces instead of chain complexes, |
|
448 |
we get a space version of an $A_\infty$ $n$-category, with $\Homeo_\bd(X)$ acting |
|
449 |
instead of $C_*(\Homeo_\bd(X))$. |
|
450 |
Taking singular chains converts a space-type $A_\infty$ $n$-category into a chain complex |
|
451 |
type $A_\infty$ $n$-category. |
|
452 |
||
99 | 453 |
\medskip |
97 | 454 |
|
99 | 455 |
The alert reader will have already noticed that our definition of (plain) $n$-category |
456 |
is extremely similar to our definition of topological fields. |
|
142 | 457 |
The main difference is that for the $n$-category definition we restrict our attention to balls |
99 | 458 |
(and their boundaries), while for fields we consider all manifolds. |
142 | 459 |
(A minor difference is that in the category definition we directly impose isotopy |
460 |
invariance in dimension $n$, while in the fields definition we have non-isotopy-invariant fields |
|
461 |
but then mod out by local relations which imply isotopy invariance.) |
|
99 | 462 |
Thus a system of fields determines an $n$-category simply by restricting our attention to |
463 |
balls. |
|
142 | 464 |
This $n$-category can be thought of as the local part of the fields. |
99 | 465 |
Conversely, given an $n$-category we can construct a system of fields via |
191
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466 |
a colimit construction; see \S \ref{ss:ncat_fields} below. |
99 | 467 |
|
142 | 468 |
%\nn{Next, say something about $A_\infty$ $n$-categories and ``homological" systems |
469 |
%of fields. |
|
470 |
%The universal (colimit) construction becomes our generalized definition of blob homology. |
|
471 |
%Need to explain how it relates to the old definition.} |
|
97 | 472 |
|
95 | 473 |
\medskip |
474 |
||
195 | 475 |
\subsection{Examples of $n$-categories}\ \ |
190
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|
101 | 477 |
\nn{these examples need to be fleshed out a bit more} |
478 |
||
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479 |
We now describe several classes of examples of $n$-categories satisfying our axioms. |
101 | 480 |
|
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481 |
\begin{example}[Maps to a space] |
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482 |
\rm |
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\label{ex:maps-to-a-space}% |
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484 |
Fix a `target space' $T$, any topological space. We define $\pi_{\leq n}(T)$, the fundamental $n$-category of $T$, as follows. |
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For $X$ a $k$-ball or $k$-sphere with $k < n$, define $\pi_{\leq n}(T)(X)$ to be the set of |
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486 |
all continuous maps from $X$ to $T$. |
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For $X$ an $n$-ball define $\pi_{\leq n}(T)(X)$ to be continuous maps from $X$ to $T$ modulo |
196 | 488 |
homotopies fixed on $\bd X$. |
101 | 489 |
(Note that homotopy invariance implies isotopy invariance.) |
490 |
For $a\in \cC(X)$ define the product morphism $a\times D \in \cC(X\times D)$ to |
|
491 |
be $a\circ\pi_X$, where $\pi_X : X\times D \to X$ is the projection. |
|
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492 |
\end{example} |
101 | 493 |
|
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494 |
\begin{example}[Maps to a space, with a fiber] |
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495 |
\rm |
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496 |
\label{ex:maps-to-a-space-with-a-fiber}% |
196 | 497 |
We can modify the example above, by fixing a |
498 |
closed $m$-manifold $F$, and defining $\pi^{\times F}_{\leq n}(T)(X) = \Maps(X \times F \to T)$, otherwise leaving the definition in Example \ref{ex:maps-to-a-space} unchanged. Taking $F$ to be a point recovers the previous case. |
|
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499 |
\end{example} |
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500 |
|
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\begin{example}[Linearized, twisted, maps to a space] |
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502 |
\rm |
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\label{ex:linearized-maps-to-a-space}% |
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504 |
We can linearize Examples \ref{ex:maps-to-a-space} and \ref{ex:maps-to-a-space-with-a-fiber} as follows. |
101 | 505 |
Let $\alpha$ be an $(n{+}m{+}1)$-cocycle on $T$ with values in a ring $R$ |
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506 |
(have in mind the trivial cocycle). |
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For $X$ of dimension less than $n$ define $\pi^{\alpha, \times F}_{\leq n}(T)(X)$ as before, ignoring $\alpha$. |
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For $X$ an $n$-ball and $c\in \Maps(\bdy X \times F \to T)$ define $\pi^{\alpha, \times F}_{\leq n}(T)(X; c)$ to be |
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509 |
the $R$-module of finite linear combinations of continuous maps from $X\times F$ to $T$, |
101 | 510 |
modulo the relation that if $a$ is homotopic to $b$ (rel boundary) via a homotopy |
191
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511 |
$h: X\times F\times I \to T$, then $a = \alpha(h)b$. |
101 | 512 |
\nn{need to say something about fundamental classes, or choose $\alpha$ carefully} |
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513 |
\end{example} |
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514 |
|
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515 |
The next example is only intended to be illustrative, as we don't specify which definition of a `traditional $n$-category' we intend. Further, most of these definitions don't even have an agreed-upon notion of `strong duality', which we assume here. |
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516 |
\begin{example}[Traditional $n$-categories] |
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517 |
\rm |
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518 |
\label{ex:traditional-n-categories} |
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519 |
Given a `traditional $n$-category with strong duality' $C$ |
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520 |
define $\cC(X)$, for $X$ a $k$-ball or $k$-sphere with $k < n$, |
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to be the set of all $C$-labeled sub cell complexes of $X$. |
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522 |
(See Subsection \ref{sec:fields}.) |
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523 |
For $X$ an $n$-ball and $c\in \cC(\bd X)$, define $\cC(X)$ to finite linear |
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524 |
combinations of $C$-labeled sub cell complexes of $X$ |
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525 |
modulo the kernel of the evaluation map. |
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526 |
Define a product morphism $a\times D$, for $D$ an $m$-ball, to be the product of the cell complex of $a$ with $D$, |
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527 |
with each cell labelled by the $m$-th iterated identity morphism of the corresponding cell for $a$. |
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528 |
More generally, start with an $n{+}m$-category $C$ and a closed $m$-manifold $F$. |
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529 |
Define $\cC(X)$, for $\dim(X) < n$, |
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to be the set of all $C$-labeled sub cell complexes of $X\times F$. |
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531 |
Define $\cC(X; c)$, for $X$ an $n$-ball, |
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532 |
to be the dual Hilbert space $A(X\times F; c)$. |
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533 |
\nn{refer elsewhere for details?} |
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534 |
\end{example} |
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535 |
|
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536 |
Finally, we describe a version of the bordism $n$-category suitable to our definitions. |
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537 |
\newcommand{\Bord}{\operatorname{Bord}} |
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538 |
\begin{example}[The bordism $n$-category] |
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539 |
\rm |
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540 |
\label{ex:bordism-category} |
196 | 541 |
For a $k$-ball or $k$-sphere $X$, $k<n$, define $\Bord^n(X)$ to be the set of all $k$-dimensional |
191
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542 |
submanifolds $W$ of $X\times \Real^\infty$ such that the projection $W \to X$ is transverse |
196 | 543 |
to $\bd X$. |
191
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544 |
For an $n$-ball $X$ define $\Bord^n(X)$ to be homeomorphism classes (rel boundary) of such submanifolds; |
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545 |
we identify $W$ and $W'$ if $\bd W = \bd W'$ and there is a homeomorphism |
196 | 546 |
$W \to W'$ which restricts to the identity on the boundary. |
191
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547 |
\end{example} |
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548 |
|
196 | 549 |
%\nn{the next example might be an unnecessary distraction. consider deleting it.} |
101 | 550 |
|
196 | 551 |
%\begin{example}[Variation on the above examples] |
552 |
%We could allow $F$ to have boundary and specify boundary conditions on $X\times \bd F$, |
|
553 |
%for example product boundary conditions or take the union over all boundary conditions. |
|
554 |
%%\nn{maybe should not emphasize this case, since it's ``better" in some sense |
|
555 |
%%to think of these guys as affording a representation |
|
556 |
%%of the $n{+}1$-category associated to $\bd F$.} |
|
557 |
%\end{example} |
|
101 | 558 |
|
559 |
||
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560 |
We have two main examples of $A_\infty$ $n$-categories, coming from maps to a target space and from the blob complex. |
101 | 561 |
|
191
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562 |
\begin{example}[Chains of maps to a space] |
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563 |
\rm |
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564 |
\label{ex:chains-of-maps-to-a-space} |
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565 |
We can modify Example \ref{ex:maps-to-a-space} above to define the fundamental $A_\infty$ $n$-category $\pi^\infty_{\le n}(T)$ of a topological space $T$. |
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566 |
For $k$-balls and $k$-spheres $X$, with $k < n$, the sets $\pi^\infty_{\leq n}(T)(X)$ are just $\Maps{X \to T}$. |
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567 |
Define $\pi^\infty_{\leq n}(T)(X; c)$ for an $n$-ball $X$ and $c \in \pi^\infty_{\leq n}(T)(\bdy X)$ to be the chain complex |
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568 |
$$C_*(\Maps_c(X\times F \to T)),$$ where $\Maps_c$ denotes continuous maps restricting to $c$ on the boundary, |
101 | 569 |
and $C_*$ denotes singular chains. |
190
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570 |
\end{example} |
101 | 571 |
|
191
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572 |
See ??? below, recovering $C_*(\Maps{M \to T})$ as (up to homotopy) the blob complex of $M$ with coefficients in $\pi^\infty_{\le n}(T)$. |
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573 |
|
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574 |
\begin{example}[Blob complexes of balls (with a fiber)] |
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575 |
\rm |
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576 |
\label{ex:blob-complexes-of-balls} |
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577 |
Fix an $m$-dimensional manifold $F$. |
101 | 578 |
Given a plain $n$-category $C$, |
190
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579 |
when $X$ is a $k$-ball or $k$-sphere, with $k<n-m$, define $\cC(X) = C(X)$. When $X$ is an $(n-m)$-ball, |
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580 |
define $\cC(X; c) = \bc^C_*(X\times F; c)$ |
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581 |
where $\bc^C_*$ denotes the blob complex based on $C$. |
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|
582 |
\end{example} |
101 | 583 |
|
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584 |
This example will be essential for ???, which relates ... |
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585 |
|
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586 |
\begin{example} |
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587 |
\nn{should add $\infty$ version of bordism $n$-cat} |
191
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588 |
\end{example} |
95 | 589 |
|
108 | 590 |
|
591 |
||
592 |
||
593 |
||
594 |
||
595 |
\subsection{From $n$-categories to systems of fields} |
|
113 | 596 |
\label{ss:ncat_fields} |
197 | 597 |
In this section we describe how to extend an $n$-category as described above (of either the plain or $A_\infty$ variety) to a system of fields. That is, we show that functors $\cC_k$ satisfying the axioms above have a canonical extension, from $k$-balls and $k$-spheres to arbitrary $k$-manifolds. |
108 | 598 |
|
197 | 599 |
We will first define the `cell-decomposition' poset $\cJ(W)$ for any $k$-manifold $W$, for $1 \leq k \leq n$. |
600 |
An $n$-category $\cC$ provides a functor from this poset to the category of sets, and we will define $\cC(W)$ as a suitable colimit (or homotopy colimit in the $A_\infty$ case) of this functor. |
|
601 |
We'll later give a more explicit description of this colimit. In the case that the $n$-category $\cC$ is enriched (e.g. associates vector spaces or chain complexes to $n$-manifolds with boundary data), then the resulting system of fields is also enriched, that is, the set associated to $W$ splits into subsets according to boundary data, and each of these subsets has the appropriate structure (e.g. a vector space or chain complex). |
|
108 | 602 |
|
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603 |
\begin{defn} |
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604 |
Say that a `permissible decomposition' of $W$ is a cell decomposition |
108 | 605 |
\[ |
606 |
W = \bigcup_a X_a , |
|
607 |
\] |
|
142 | 608 |
where each closed top-dimensional cell $X_a$ is an embedded $k$-ball. |
191
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609 |
|
108 | 610 |
Given permissible decompositions $x$ and $y$, we say that $x$ is a refinement |
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611 |
of $y$, or write $x \le y$, if each $k$-ball of $y$ is a union of $k$-balls of $x$. |
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612 |
|
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613 |
The category $\cJ(W)$ has objects the permissible decompositions of $W$, and a unique morphism from $x$ to $y$ if and only if $x$ is a refinement of $y$. |
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614 |
See Figure \ref{partofJfig} for an example. |
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615 |
\end{defn} |
119 | 616 |
|
617 |
\begin{figure}[!ht] |
|
618 |
\begin{equation*} |
|
619 |
\mathfig{.63}{tempkw/zz2} |
|
620 |
\end{equation*} |
|
621 |
\caption{A small part of $\cJ(W)$} |
|
622 |
\label{partofJfig} |
|
623 |
\end{figure} |
|
624 |
||
108 | 625 |
|
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626 |
|
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627 |
An $n$-category $\cC$ determines |
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628 |
a functor $\psi_{\cC;W}$ from $\cJ(W)$ to the category of sets |
108 | 629 |
(possibly with additional structure if $k=n$). |
197 | 630 |
Each $k$-ball $X$ of a decomposition $y$ of $W$ has its boundary decomposed into $k{-}1$-balls, |
631 |
and, as described above, we have a subset $\cC(X)\spl \sub \cC(X)$ of morphisms whose boundaries |
|
632 |
are splittable along this decomposition. |
|
633 |
%For a $k$-cell $X$ in a cell composition of $W$, we can consider the `splittable fields' $\cC(X)_{\bdy X}$, the subset of $\cC(X)$ consisting of fields which are splittable with respect to each boundary $k-1$-cell. |
|
108 | 634 |
|
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635 |
\begin{defn} |
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636 |
Define the functor $\psi_{\cC;W} : \cJ(W) \to \Set$ as follows. |
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637 |
For a decomposition $x = \bigcup_a X_a$ in $\cJ(W)$, $\psi_{\cC;W}(x)$ is the subset |
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638 |
\begin{equation} |
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639 |
\label{eq:psi-C} |
197 | 640 |
\psi_{\cC;W}(x) \sub \prod_a \cC(X_a)\spl |
191
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641 |
\end{equation} |
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642 |
where the restrictions to the various pieces of shared boundaries amongst the cells |
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643 |
$X_a$ all agree (this is a fibered product of all the labels of $n$-cells over the labels of $n-1$-cells). |
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644 |
If $x$ is a refinement of $y$, the map $\psi_{\cC;W}(x) \to \psi_{\cC;W}(y)$ is given by the composition maps of $\cC$. |
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645 |
\end{defn} |
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|
646 |
|
197 | 647 |
When the $n$-category $\cC$ is enriched in some monoidal category $(A,\boxtimes)$, and $W$ is a |
648 |
closed $n$-manifold, the functor $\psi_{\cC;W}$ has target $A$ and |
|
191
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|
649 |
we replace the cartesian product of sets appearing in Equation \eqref{eq:psi-C} with the monoidal product $\boxtimes$. (Moreover, $\psi_{\cC;W}(x)$ might be a subobject, rather than a subset, of the product.) |
197 | 650 |
Similar things are true if $W$ is an $n$-manifold with non-empty boundary and we |
651 |
fix a field on $\bd W$ |
|
652 |
(i.e. fix an element of the colimit associated to $\bd W$). |
|
191
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653 |
|
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654 |
Finally, we construct $\cC(W)$ as the appropriate colimit of $\psi_{\cC;W}$. |
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655 |
|
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656 |
\begin{defn}[System of fields functor] |
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657 |
If $\cC$ is an $n$-category enriched in sets or vector spaces, $\cC(W)$ is the usual colimit of the functor $\psi_{\cC;W}$. |
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658 |
That is, for each decomposition $x$ there is a map |
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659 |
$\psi_{\cC;W}(x)\to \cC(W)$, these maps are compatible with the refinement maps |
108 | 660 |
above, and $\cC(W)$ is universal with respect to these properties. |
191
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661 |
\end{defn} |
112 | 662 |
|
191
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663 |
\begin{defn}[System of fields functor, $A_\infty$ case] |
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|
664 |
When $\cC$ is an $A_\infty$ $n$-category, $\cC(W)$ for $W$ a $k$-manifold with $k < n$ is defined as above, as the colimit of $\psi_{\cC;W}$. When $W$ is an $n$-manifold, the chain complex $\cC(W)$ is the homotopy colimit of the functor $\psi_{\cC;W}$. |
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665 |
\end{defn} |
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666 |
|
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667 |
We can specify boundary data $c \in \cC(\bdy W)$, and define functors $\psi_{\cC;W,c}$ with values the subsets of those of $\psi_{\cC;W}$ which agree with $c$ on the boundary of $W$. |
111 | 668 |
|
197 | 669 |
We now give a more concrete description of the colimit in each case. If $\cC$ is enriched over vector spaces, and $W$ is an $n$-manifold, we can take the vector space $\cC(W,c)$ to be the direct sum over all permissible decompositions of $W$ |
191
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|
670 |
\begin{equation*} |
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|
671 |
\cC(W,c) = \left( \bigoplus_x \psi_{\cC;W,c}(x)\right) \big/ K |
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|
672 |
\end{equation*} |
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673 |
where $K$ is the vector space spanned by elements $a - g(a)$, with |
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|
674 |
$a\in \psi_{\cC;W,c}(x)$ for some decomposition $x$, and $g: \psi_{\cC;W,c}(x) |
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|
675 |
\to \psi_{\cC;W,c}(y)$ is value of $\psi_{\cC;W,c}$ on some antirefinement $x \leq y$. |
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|
676 |
|
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|
677 |
In the $A_\infty$ case enriched over chain complexes, the concrete description of the homotopy colimit |
197 | 678 |
is more involved. |
142 | 679 |
%\nn{should probably rewrite this to be compatible with some standard reference} |
191
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680 |
Define an $m$-sequence in $W$ to be a sequence $x_0 \le x_1 \le \dots \le x_m$ of permissible decompositions of $W$. |
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681 |
Such sequences (for all $m$) form a simplicial set in $\cJ(W)$. |
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|
682 |
Define $V$ as a vector space via |
112 | 683 |
\[ |
191
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|
684 |
V = \bigoplus_{(x_i)} \psi_{\cC;W}(x_0)[m] , |
112 | 685 |
\] |
198 | 686 |
where the sum is over all $m$-sequences $(x_i)$ and all $m$, and each summand is degree shifted by $m$. (Our homological conventions are non-standard: if a complex $U$ is concentrated in degree $0$, the complex $U[m]$ is concentrated in degree $m$.) |
191
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|
687 |
We endow $V$ with a differential which is the sum of the differential of the $\psi_{\cC;W}(x_0)$ |
112 | 688 |
summands plus another term using the differential of the simplicial set of $m$-sequences. |
689 |
More specifically, if $(a, \bar{x})$ denotes an element in the $\bar{x}$ |
|
690 |
summand of $V$ (with $\bar{x} = (x_0,\dots,x_k)$), define |
|
691 |
\[ |
|
191
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|
692 |
\bd (a, \bar{x}) = (\bd a, \bar{x}) + (-1)^{\deg{a}} (g(a), d_0(\bar{x})) + (-1)^{\deg{a}} \sum_{j=1}^k (-1)^{j} (a, d_j(\bar{x})) , |
112 | 693 |
\] |
694 |
where $d_j(\bar{x}) = (x_0,\dots,x_{j-1},x_{j+1},\dots,x_k)$ and $g: \psi_\cC(x_0)\to \psi_\cC(x_1)$ |
|
198 | 695 |
is the usual gluing map coming from the antirefinement $x_0 \le x_1$. |
112 | 696 |
\nn{need to say this better} |
697 |
\nn{maybe mention that there is a version that emphasizes minimal gluings (antirefinements) which |
|
698 |
combine only two balls at a time; for $n=1$ this version will lead to usual definition |
|
699 |
of $A_\infty$ category} |
|
108 | 700 |
|
113 | 701 |
We will call $m$ the filtration degree of the complex. |
702 |
We can think of this construction as starting with a disjoint copy of a complex for each |
|
703 |
permissible decomposition (filtration degree 0). |
|
704 |
Then we glue these together with mapping cylinders coming from gluing maps |
|
705 |
(filtration degree 1). |
|
198 | 706 |
Then we kill the extra homology we just introduced with mapping cylinders between the mapping cylinders (filtration degree 2). |
113 | 707 |
And so on. |
708 |
||
108 | 709 |
$\cC(W)$ is functorial with respect to homeomorphisms of $k$-manifolds. |
710 |
||
711 |
It is easy to see that |
|
712 |
there are well-defined maps $\cC(W)\to\cC(\bd W)$, and that these maps |
|
713 |
comprise a natural transformation of functors. |
|
714 |
||
715 |
\nn{need to finish explaining why we have a system of fields; |
|
716 |
need to say more about ``homological" fields? |
|
717 |
(actions of homeomorphisms); |
|
718 |
define $k$-cat $\cC(\cdot\times W)$} |
|
719 |
||
720 |
||
721 |
||
722 |
\subsection{Modules} |
|
95 | 723 |
|
101 | 724 |
Next we define [$A_\infty$] $n$-category modules (a.k.a.\ representations, |
725 |
a.k.a.\ actions). |
|
199 | 726 |
The definition will be very similar to that of $n$-categories, |
727 |
but with $k$-balls replaced by {\it marked $k$-balls,} defined below. |
|
109 | 728 |
\nn{** need to make sure all revisions of $n$-cat def are also made to module def.} |
198 | 729 |
%\nn{should they be called $n$-modules instead of just modules? probably not, but worth considering.} |
730 |
||
104 | 731 |
Our motivating example comes from an $(m{-}n{+}1)$-dimensional manifold $W$ with boundary |
102 | 732 |
in the context of an $m{+}1$-dimensional TQFT. |
733 |
Such a $W$ gives rise to a module for the $n$-category associated to $\bd W$. |
|
734 |
This will be explained in more detail as we present the axioms. |
|
735 |
||
736 |
Fix an $n$-category $\cC$. |
|
737 |
||
738 |
Define a {\it marked $k$-ball} to be a pair $(B, N)$ homeomorphic to the pair |
|
739 |
(standard $k$-ball, northern hemisphere in boundary of standard $k$-ball). |
|
740 |
We call $B$ the ball and $N$ the marking. |
|
741 |
A homeomorphism between marked $k$-balls is a homeomorphism of balls which |
|
742 |
restricts to a homeomorphism of markings. |
|
743 |
||
199 | 744 |
\mmpar{Module axiom 1}{Module morphisms} |
102 | 745 |
{For each $0 \le k \le n$, we have a functor $\cM_k$ from |
746 |
the category of marked $k$-balls and |
|
747 |
homeomorphisms to the category of sets and bijections.} |
|
748 |
||
749 |
(As with $n$-categories, we will usually omit the subscript $k$.) |
|
750 |
||
104 | 751 |
For example, let $\cD$ be the $m{+}1$-dimensional TQFT which assigns to a $k$-manifold $N$ the set |
752 |
of maps from $N$ to $T$, modulo homotopy (and possibly linearized) if $k=m$. |
|
753 |
Let $W$ be an $(m{-}n{+}1)$-dimensional manifold with boundary. |
|
754 |
Let $\cC$ be the $n$-category with $\cC(X) \deq \cD(X\times \bd W)$. |
|
755 |
Let $\cM(B, N) \deq \cD((B\times \bd W)\cup (N\times W))$. |
|
756 |
(The union is along $N\times \bd W$.) |
|
110 | 757 |
(If $\cD$ were a general TQFT, we would define $\cM(B, N)$ to be |
758 |
the subset of $\cD((B\times \bd W)\cup (N\times W))$ which is splittable along $N\times \bd W$.) |
|
102 | 759 |
|
182 | 760 |
\begin{figure}[!ht] |
761 |
$$\mathfig{.8}{tempkw/blah15}$$ |
|
762 |
\caption{From manifold with boundary collar to marked ball}\label{blah15}\end{figure} |
|
763 |
||
103 | 764 |
Define the boundary of a marked $k$-ball $(B, N)$ to be the pair $(\bd B \setmin N, \bd N)$. |
765 |
Call such a thing a {marked $k{-}1$-hemisphere}. |
|
102 | 766 |
|
199 | 767 |
\mmpar{Module axiom 2}{Module boundaries (hemispheres)} |
102 | 768 |
{For each $0 \le k \le n-1$, we have a functor $\cM_k$ from |
104 | 769 |
the category of marked $k$-hemispheres and |
102 | 770 |
homeomorphisms to the category of sets and bijections.} |
771 |
||
104 | 772 |
In our example, let $\cM(H) \deq \cD(H\times\bd W \cup \bd H\times W)$. |
773 |
||
199 | 774 |
\mmpar{Module axiom 3}{Module boundaries (maps)} |
102 | 775 |
{For each marked $k$-ball $M$ we have a map of sets $\bd: \cM(M)\to \cM(\bd M)$. |
776 |
These maps, for various $M$, comprise a natural transformation of functors.} |
|
777 |
||
110 | 778 |
Given $c\in\cM(\bd M)$, let $\cM(M; c) \deq \bd^{-1}(c)$. |
102 | 779 |
|
780 |
If the $n$-category $\cC$ is enriched over some other category (e.g.\ vector spaces), |
|
781 |
then $\cM(M; c)$ should be an object in that category for each marked $n$-ball $M$ |
|
782 |
and $c\in \cC(\bd M)$. |
|
783 |
||
199 | 784 |
\mmpar{Module axiom 4}{Boundary from domain and range} |
102 | 785 |
{Let $H = M_1 \cup_E M_2$, where $H$ is a marked $k$-hemisphere ($0\le k\le n-1$), |
104 | 786 |
$M_i$ is a marked $k$-ball, and $E = M_1\cap M_2$ is a marked $k{-}1$-hemisphere. |
787 |
Let $\cM(M_1) \times_{\cM(E)} \cM(M_2)$ denote the fibered product of the |
|
788 |
two maps $\bd: \cM(M_i)\to \cM(E)$. |
|
102 | 789 |
Then (axiom) we have an injective map |
790 |
\[ |
|
199 | 791 |
\gl_E : \cM(M_1) \times_{\cM(E)} \cM(M_2) \hookrightarrow \cM(H) |
102 | 792 |
\] |
793 |
which is natural with respect to the actions of homeomorphisms.} |
|
794 |
||
110 | 795 |
Let $\cM(H)_E$ denote the image of $\gl_E$. |
796 |
We will refer to elements of $\cM(H)_E$ as ``splittable along $E$" or ``transverse to $E$". |
|
797 |
||
798 |
||
199 | 799 |
\mmpar{Module axiom 5}{Module to category restrictions} |
103 | 800 |
{For each marked $k$-hemisphere $H$ there is a restriction map |
801 |
$\cM(H)\to \cC(H)$. |
|
802 |
($\cC(H)$ means apply $\cC$ to the underlying $k$-ball of $H$.) |
|
803 |
These maps comprise a natural transformation of functors.} |
|
102 | 804 |
|
103 | 805 |
Note that combining the various boundary and restriction maps above |
110 | 806 |
(for both modules and $n$-categories) |
103 | 807 |
we have for each marked $k$-ball $(B, N)$ and each $k{-}1$-ball $Y\sub \bd B \setmin N$ |
808 |
a natural map from a subset of $\cM(B, N)$ to $\cC(Y)$. |
|
110 | 809 |
The subset is the subset of morphisms which are appropriately splittable (transverse to the |
810 |
cutting submanifolds). |
|
103 | 811 |
This fact will be used below. |
102 | 812 |
|
104 | 813 |
In our example, the various restriction and gluing maps above come from |
814 |
restricting and gluing maps into $T$. |
|
815 |
||
816 |
We require two sorts of composition (gluing) for modules, corresponding to two ways |
|
103 | 817 |
of splitting a marked $k$-ball into two (marked or plain) $k$-balls. |
119 | 818 |
(See Figure \ref{zzz3}.) |
103 | 819 |
|
119 | 820 |
\begin{figure}[!ht] |
821 |
\begin{equation*} |
|
822 |
\mathfig{.63}{tempkw/zz3} |
|
823 |
\end{equation*} |
|
824 |
\caption{Module composition (top); $n$-category action (bottom)} |
|
825 |
\label{zzz3} |
|
826 |
\end{figure} |
|
827 |
||
828 |
First, we can compose two module morphisms to get another module morphism. |
|
103 | 829 |
|
830 |
\xxpar{Module composition:} |
|
831 |
{Let $M = M_1 \cup_Y M_2$, where $M$, $M_1$ and $M_2$ are marked $k$-balls ($0\le k\le n$) |
|
832 |
and $Y = M_1\cap M_2$ is a marked $k{-}1$-ball. |
|
833 |
Let $E = \bd Y$, which is a marked $k{-}2$-hemisphere. |
|
834 |
Note that each of $M$, $M_1$ and $M_2$ has its boundary split into two marked $k{-}1$-balls by $E$. |
|
835 |
We have restriction (domain or range) maps $\cM(M_i)_E \to \cM(Y)$. |
|
836 |
Let $\cM(M_1)_E \times_{\cM(Y)} \cM(M_2)_E$ denote the fibered product of these two maps. |
|
837 |
Then (axiom) we have a map |
|
838 |
\[ |
|
839 |
\gl_Y : \cM(M_1)_E \times_{\cM(Y)} \cM(M_2)_E \to \cM(M)_E |
|
840 |
\] |
|
841 |
which is natural with respect to the actions of homeomorphisms, and also compatible with restrictions |
|
842 |
to the intersection of the boundaries of $M$ and $M_i$. |
|
843 |
If $k < n$ we require that $\gl_Y$ is injective. |
|
844 |
(For $k=n$, see below.)} |
|
845 |
||
119 | 846 |
|
847 |
||
103 | 848 |
Second, we can compose an $n$-category morphism with a module morphism to get another |
849 |
module morphism. |
|
850 |
We'll call this the action map to distinguish it from the other kind of composition. |
|
851 |
||
852 |
\xxpar{$n$-category action:} |
|
853 |
{Let $M = X \cup_Y M'$, where $M$ and $M'$ are marked $k$-balls ($0\le k\le n$), |
|
854 |
$X$ is a plain $k$-ball, |
|
855 |
and $Y = X\cap M'$ is a $k{-}1$-ball. |
|
856 |
Let $E = \bd Y$, which is a $k{-}2$-sphere. |
|
857 |
We have restriction maps $\cM(M')_E \to \cC(Y)$ and $\cC(X)_E\to \cC(Y)$. |
|
858 |
Let $\cC(X)_E \times_{\cC(Y)} \cM(M')_E$ denote the fibered product of these two maps. |
|
859 |
Then (axiom) we have a map |
|
860 |
\[ |
|
861 |
\gl_Y :\cC(X)_E \times_{\cC(Y)} \cM(M')_E \to \cM(M)_E |
|
862 |
\] |
|
863 |
which is natural with respect to the actions of homeomorphisms, and also compatible with restrictions |
|
864 |
to the intersection of the boundaries of $X$ and $M'$. |
|
865 |
If $k < n$ we require that $\gl_Y$ is injective. |
|
866 |
(For $k=n$, see below.)} |
|
867 |
||
868 |
\xxpar{Module strict associativity:} |
|
869 |
{The composition and action maps above are strictly associative.} |
|
870 |
||
110 | 871 |
Note that the above associativity axiom applies to mixtures of module composition, |
872 |
action maps and $n$-category composition. |
|
119 | 873 |
See Figure \ref{zzz1b}. |
874 |
||
875 |
\begin{figure}[!ht] |
|
876 |
\begin{equation*} |
|
877 |
\mathfig{1}{tempkw/zz1b} |
|
878 |
\end{equation*} |
|
879 |
\caption{Two examples of mixed associativity} |
|
880 |
\label{zzz1b} |
|
881 |
\end{figure} |
|
882 |
||
110 | 883 |
|
884 |
The above three axioms are equivalent to the following axiom, |
|
103 | 885 |
which we state in slightly vague form. |
886 |
\nn{need figure for this} |
|
887 |
||
888 |
\xxpar{Module multi-composition:} |
|
889 |
{Given any decomposition |
|
890 |
\[ |
|
891 |
M = X_1 \cup\cdots\cup X_p \cup M_1\cup\cdots\cup M_q |
|
892 |
\] |
|
893 |
of a marked $k$-ball $M$ |
|
894 |
into small (marked and plain) $k$-balls $M_i$ and $X_j$, there is a |
|
895 |
map from an appropriate subset (like a fibered product) |
|
896 |
of |
|
897 |
\[ |
|
898 |
\cC(X_1)\times\cdots\times\cC(X_p) \times \cM(M_1)\times\cdots\times\cM(M_q) |
|
899 |
\] |
|
900 |
to $\cM(M)$, |
|
901 |
and these various multifold composition maps satisfy an |
|
902 |
operad-type strict associativity condition.} |
|
903 |
||
904 |
(The above operad-like structure is analogous to the swiss cheese operad |
|
146 | 905 |
\cite{MR1718089}.) |
103 | 906 |
\nn{need to double-check that this is true.} |
907 |
||
908 |
\xxpar{Module product (identity) morphisms:} |
|
909 |
{Let $M$ be a marked $k$-ball and $D$ be a plain $m$-ball, with $k+m \le n$. |
|
910 |
Then we have a map $\cM(M)\to \cM(M\times D)$, usually denoted $a\mapsto a\times D$ for $a\in \cM(M)$. |
|
911 |
If $f:M\to M'$ and $\tilde{f}:M\times D \to M'\times D'$ are maps such that the diagram |
|
912 |
\[ \xymatrix{ |
|
913 |
M\times D \ar[r]^{\tilde{f}} \ar[d]_{\pi} & M'\times D' \ar[d]^{\pi} \\ |
|
914 |
M \ar[r]^{f} & M' |
|
915 |
} \] |
|
916 |
commutes, then we have $\tilde{f}(a\times D) = f(a)\times D'$.} |
|
917 |
||
111 | 918 |
\nn{Need to add compatibility with various things, as in the n-cat version of this axiom above.} |
103 | 919 |
|
110 | 920 |
\nn{** marker --- resume revising here **} |
921 |
||
103 | 922 |
There are two alternatives for the next axiom, according whether we are defining |
923 |
modules for plain $n$-categories or $A_\infty$ $n$-categories. |
|
924 |
In the plain case we require |
|
925 |
||
185 | 926 |
\xxpar{Extended isotopy invariance in dimension $n$:} |
103 | 927 |
{Let $M$ be a marked $n$-ball and $f: M\to M$ be a homeomorphism which restricts |
175 | 928 |
to the identity on $\bd M$ and is extended isotopic (rel boundary) to the identity. |
103 | 929 |
Then $f$ acts trivially on $\cM(M)$.} |
930 |
||
931 |
\nn{need to rephrase this, since extended isotopies don't correspond to homeomorphisms.} |
|
932 |
||
933 |
We emphasize that the $\bd M$ above means boundary in the marked $k$-ball sense. |
|
934 |
In other words, if $M = (B, N)$ then we require only that isotopies are fixed |
|
935 |
on $\bd B \setmin N$. |
|
936 |
||
937 |
For $A_\infty$ modules we require |
|
938 |
||
939 |
\xxpar{Families of homeomorphisms act.} |
|
940 |
{For each marked $n$-ball $M$ and each $c\in \cM(\bd M)$ we have a map of chain complexes |
|
941 |
\[ |
|
942 |
C_*(\Homeo_\bd(M))\ot \cM(M; c) \to \cM(M; c) . |
|
943 |
\] |
|
944 |
Here $C_*$ means singular chains and $\Homeo_\bd(M)$ is the space of homeomorphisms of $M$ |
|
945 |
which fix $\bd M$. |
|
946 |
These action maps are required to be associative up to homotopy |
|
947 |
\nn{iterated homotopy?}, and also compatible with composition (gluing) in the sense that |
|
948 |
a diagram like the one in Proposition \ref{CDprop} commutes. |
|
949 |
\nn{repeat diagram here?} |
|
950 |
\nn{restate this with $\Homeo(M\to M')$? what about boundary fixing property?}} |
|
951 |
||
952 |
\medskip |
|
102 | 953 |
|
104 | 954 |
Note that the above axioms imply that an $n$-category module has the structure |
955 |
of an $n{-}1$-category. |
|
956 |
More specifically, let $J$ be a marked 1-ball, and define $\cE(X)\deq \cM(X\times J)$, |
|
957 |
where $X$ is a $k$-ball or $k{-}1$-sphere and in the product $X\times J$ we pinch |
|
958 |
above the non-marked boundary component of $J$. |
|
959 |
\nn{give figure for this, or say more?} |
|
960 |
Then $\cE$ has the structure of an $n{-}1$-category. |
|
102 | 961 |
|
105 | 962 |
All marked $k$-balls are homeomorphic, unless $k = 1$ and our manifolds |
963 |
are oriented or Spin (but not unoriented or $\text{Pin}_\pm$). |
|
964 |
In this case ($k=1$ and oriented or Spin), there are two types |
|
965 |
of marked 1-balls, call them left-marked and right-marked, |
|
966 |
and hence there are two types of modules, call them right modules and left modules. |
|
967 |
In all other cases ($k>1$ or unoriented or $\text{Pin}_\pm$), |
|
968 |
there is no left/right module distinction. |
|
969 |
||
130 | 970 |
\medskip |
971 |
||
972 |
Examples of modules: |
|
973 |
\begin{itemize} |
|
142 | 974 |
\item \nn{examples from TQFTs} |
975 |
\item \nn{for maps to $T$, can restrict to subspaces of $T$;} |
|
130 | 976 |
\end{itemize} |
108 | 977 |
|
978 |
\subsection{Modules as boundary labels} |
|
112 | 979 |
\label{moddecss} |
108 | 980 |
|
981 |
Let $\cC$ be an [$A_\infty$] $n$-category, let $W$ be a $k$-manifold ($k\le n$), |
|
143 | 982 |
let $\{Y_i\}$ be a collection of disjoint codimension 0 submanifolds of $\bd W$, |
983 |
and let $\cN = (\cN_i)$ be an assignment of a $\cC$ module $\cN_i$ to $Y_i$. |
|
984 |
||
985 |
%Let $\cC$ be an [$A_\infty$] $n$-category, let $W$ be a $k$-manifold ($k\le n$), |
|
986 |
%and let $\cN = (\cN_i)$ be an assignment of a $\cC$ module $\cN_i$ to each boundary |
|
987 |
%component $\bd_i W$ of $W$. |
|
988 |
%(More generally, each $\cN_i$ could label some codimension zero submanifold of $\bd W$.) |
|
108 | 989 |
|
990 |
We will define a set $\cC(W, \cN)$ using a colimit construction similar to above. |
|
991 |
\nn{give ref} |
|
992 |
(If $k = n$ and our $k$-categories are enriched, then |
|
993 |
$\cC(W, \cN)$ will have additional structure; see below.) |
|
994 |
||
995 |
Define a permissible decomposition of $W$ to be a decomposition |
|
996 |
\[ |
|
191
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|
997 |
W = \left(\bigcup_a X_a\right) \cup \left(\bigcup_{i,b} M_{ib}\right) , |
108 | 998 |
\] |
999 |
where each $X_a$ is a plain $k$-ball (disjoint from $\bd W$) and |
|
1000 |
each $M_{ib}$ is a marked $k$-ball intersecting $\bd_i W$, |
|
143 | 1001 |
with $M_{ib}\cap Y_i$ being the marking. |
1002 |
(See Figure \ref{mblabel}.) |
|
1003 |
\begin{figure}[!ht]\begin{equation*} |
|
1004 |
\mathfig{.9}{tempkw/mblabel} |
|
1005 |
\end{equation*}\caption{A permissible decomposition of a manifold |
|
146 | 1006 |
whose boundary components are labeled by $\cC$ modules $\{\cN_i\}$.}\label{mblabel}\end{figure} |
108 | 1007 |
Given permissible decompositions $x$ and $y$, we say that $x$ is a refinement |
1008 |
of $y$, or write $x \le y$, if each ball of $y$ is a union of balls of $x$. |
|
1009 |
This defines a partial ordering $\cJ(W)$, which we will think of as a category. |
|
1010 |
(The objects of $\cJ(D)$ are permissible decompositions of $W$, and there is a unique |
|
1011 |
morphism from $x$ to $y$ if and only if $x$ is a refinement of $y$.) |
|
1012 |
||
1013 |
$\cN$ determines |
|
1014 |
a functor $\psi_\cN$ from $\cJ(W)$ to the category of sets |
|
1015 |
(possibly with additional structure if $k=n$). |
|
1016 |
For a decomposition $x = (X_a, M_{ib})$ in $\cJ(W)$, define $\psi_\cN(x)$ to be the subset |
|
1017 |
\[ |
|
191
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|
1018 |
\psi_\cN(x) \sub \left(\prod_a \cC(X_a)\right) \times \left(\prod_{ib} \cN_i(M_{ib})\right) |
108 | 1019 |
\] |
1020 |
such that the restrictions to the various pieces of shared boundaries amongst the |
|
1021 |
$X_a$ and $M_{ib}$ all agree. |
|
1022 |
(Think fibered product.) |
|
1023 |
If $x$ is a refinement of $y$, define a map $\psi_\cN(x)\to\psi_\cN(y)$ |
|
1024 |
via the gluing (composition or action) maps from $\cC$ and the $\cN_i$. |
|
1025 |
||
1026 |
Finally, define $\cC(W, \cN)$ to be the colimit of $\psi_\cN$. |
|
143 | 1027 |
(Recall that if $k=n$ and we are in the $A_\infty$ case, then ``colimit" means |
1028 |
homotopy colimit.) |
|
108 | 1029 |
|
143 | 1030 |
If $D$ is an $m$-ball, $0\le m \le n-k$, then we can similarly define |
1031 |
$\cC(D\times W, \cN)$, where in this case $\cN_i$ labels the submanifold |
|
1032 |
$D\times Y_i \sub \bd(D\times W)$. |
|
112 | 1033 |
|
143 | 1034 |
It is not hard to see that the assignment $D \mapsto \cT(W, \cN)(D) \deq \cC(D\times W, \cN)$ |
1035 |
has the structure of an $n{-}k$-category. |
|
144 | 1036 |
|
1037 |
\medskip |
|
1038 |
||
1039 |
||
1040 |
%\subsection{Tensor products} |
|
108 | 1041 |
|
144 | 1042 |
We will use a simple special case of the above |
1043 |
construction to define tensor products |
|
1044 |
of modules. |
|
1045 |
Let $\cM_1$ and $\cM_2$ be modules for an $n$-category $\cC$. |
|
1046 |
(If $k=1$ and manifolds are oriented, then one should be |
|
1047 |
a left module and the other a right module.) |
|
1048 |
Choose a 1-ball $J$, and label the two boundary points of $J$ by $\cM_1$ and $\cM_2$. |
|
1049 |
Define the tensor product of $\cM_1$ and $\cM_2$ to be the |
|
1050 |
$n{-}1$-category $\cT(J, \cM_1, \cM_2)$, |
|
1051 |
\[ |
|
1052 |
\cM_1\otimes \cM_2 \deq \cT(J, \cM_1, \cM_2) . |
|
1053 |
\] |
|
1054 |
This of course depends (functorially) |
|
1055 |
on the choice of 1-ball $J$. |
|
105 | 1056 |
|
144 | 1057 |
We will define a more general self tensor product (categorified coend) below. |
1058 |
||
112 | 1059 |
|
144 | 1060 |
|
1061 |
||
1062 |
%\nn{what about self tensor products /coends ?} |
|
105 | 1063 |
|
108 | 1064 |
\nn{maybe ``tensor product" is not the best name?} |
1065 |
||
144 | 1066 |
%\nn{start with (less general) tensor products; maybe change this later} |
106 | 1067 |
|
107 | 1068 |
|
1069 |
||
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|
1070 |
\subsection{The $n{+}1$-category of sphere modules} |
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|
1071 |
|
155 | 1072 |
In this subsection we define an $n{+}1$-category of ``sphere modules" whose objects |
1073 |
correspond to $n$-categories. |
|
1074 |
This is a version of the familiar algebras-bimodules-intertwinors 2-category. |
|
1075 |
(Terminology: It is clearly appropriate to call an $S^0$ modules a bimodule, |
|
1076 |
since a 0-sphere has an obvious bi-ness. |
|
1077 |
This is much less true for higher dimensional spheres, |
|
1078 |
so we prefer the term ``sphere module" for the general case.) |
|
144 | 1079 |
|
107 | 1080 |
|
1081 |
||
144 | 1082 |
\nn{need to assume a little extra structure to define the top ($n+1$) part (?)} |
101 | 1083 |
|
1084 |
\medskip |
|
1085 |
\hrule |
|
1086 |
\medskip |
|
1087 |
||
95 | 1088 |
\nn{to be continued...} |
101 | 1089 |
\medskip |
98 | 1090 |
|
1091 |
||
1092 |
Stuff that remains to be done (either below or in an appendix or in a separate section or in |
|
1093 |
a separate paper): |
|
1094 |
\begin{itemize} |
|
1095 |
\item traditional $n$-cat defs (e.g. *-1-cat, pivotal 2-cat) imply our def of plain $n$-cat |
|
1096 |
\item conversely, our def implies other defs |
|
105 | 1097 |
\item do same for modules; maybe an appendix on relating topological |
1098 |
vs traditional defs, $n = 1,2$, $A_\infty$ or not, cats, modules, tensor products |
|
98 | 1099 |
\item traditional $A_\infty$ 1-cat def implies our def |
99 | 1100 |
\item ... and vice-versa (already done in appendix) |
98 | 1101 |
\item say something about unoriented vs oriented vs spin vs pin for $n=1$ (and $n=2$?) |
1102 |
\item spell out what difference (if any) Top vs PL vs Smooth makes |
|
117
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|
1103 |
\item define $n{+}1$-cat of $n$-cats (a.k.a.\ $n{+}1$-category of generalized bimodules |
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|
1104 |
a.k.a.\ $n{+}1$-category of sphere modules); discuss Morita equivalence |
130 | 1105 |
\item morphisms of modules; show that it's adjoint to tensor product |
139 | 1106 |
(need to define dual module for this) |
1107 |
\item functors |
|
98 | 1108 |
\end{itemize} |
1109 |
||
134 | 1110 |
\nn{Some salvaged paragraphs that we might want to work back in:} |
1111 |
\hrule |
|
98 | 1112 |
|
134 | 1113 |
Appendix \ref{sec:comparing-A-infty} explains the translation between this definition of an $A_\infty$ $1$-category and the usual one expressed in terms of `associativity up to higher homotopy', as in \cite{MR1854636}. (In this version of the paper, that appendix is incomplete, however.) |
1114 |
||
1115 |
The motivating example is `chains of maps to $M$' for some fixed target space $M$. This is a topological $A_\infty$ category $\Xi_M$ with $\Xi_M(J) = C_*(\Maps(J \to M))$. The gluing maps $\Xi_M(J) \tensor \Xi_M(J') \to \Xi_M(J \cup J')$ takes the product of singular chains, then glues maps to $M$ together; the associativity condition is automatically satisfied. The evaluation map $\ev_{J,J'} : \CD{J \to J'} \tensor \Xi_M(J) \to \Xi_M(J')$ is the composition |
|
1116 |
\begin{align*} |
|
1117 |
\CD{J \to J'} \tensor C_*(\Maps(J \to M)) & \to C_*(\Diff(J \to J') \times \Maps(J \to M)) \\ & \to C_*(\Maps(J' \to M)), |
|
1118 |
\end{align*} |
|
1119 |
where the first map is the product of singular chains, and the second is precomposition by the inverse of a diffeomorphism. |
|
1120 |
||
1121 |
We now give two motivating examples, as theorems constructing other homological systems of fields, |
|
1122 |
||
1123 |
||
1124 |
\begin{thm} |
|
1125 |
For a fixed target space $X$, `chains of maps to $X$' is a homological system of fields $\Xi$, defined as |
|
1126 |
\begin{equation*} |
|
1127 |
\Xi(M) = \CM{M}{X}. |
|
1128 |
\end{equation*} |
|
1129 |
\end{thm} |
|
1130 |
||
1131 |
\begin{thm} |
|
1132 |
Given an $n$-dimensional system of fields $\cF$, and a $k$-manifold $F$, there is an $n-k$ dimensional homological system of fields $\cF^{\times F}$ defined by |
|
1133 |
\begin{equation*} |
|
1134 |
\cF^{\times F}(M) = \cB_*(M \times F, \cF). |
|
1135 |
\end{equation*} |
|
1136 |
\end{thm} |
|
1137 |
We might suggestively write $\cF^{\times F}$ as $\cB_*(F \times [0,1]^b, \cF)$, interpreting this as an (undefined!) $A_\infty$ $b$-category, and then as the resulting homological system of fields, following a recipe analogous to that given above for $A_\infty$ $1$-categories. |
|
1138 |
||
1139 |
||
1140 |
In later sections, we'll prove the following two unsurprising theorems, about the (as-yet-undefined) blob homology of these homological systems of fields. |
|
1141 |
||
1142 |
||
1143 |
\begin{thm} |
|
1144 |
\begin{equation*} |
|
1145 |
\cB_*(M, \Xi) \iso \Xi(M) |
|
1146 |
\end{equation*} |
|
1147 |
\end{thm} |
|
1148 |
||
1149 |
\begin{thm}[Product formula] |
|
1150 |
Given a $b$-manifold $B$, an $f$-manifold $F$ and a $b+f$ dimensional system of fields, |
|
1151 |
there is a quasi-isomorphism |
|
1152 |
\begin{align*} |
|
1153 |
\cB_*(B \times F, \cF) & \quismto \cB_*(B, \cF^{\times F}) |
|
1154 |
\end{align*} |
|
1155 |
\end{thm} |
|
1156 |
||
1157 |
\begin{question} |
|
1158 |
Is it possible to compute the blob homology of a non-trivial bundle in terms of the blob homology of its fiber? |
|
1159 |
\end{question} |
|
1160 |
||
1161 |
\hrule |