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%!TEX root = ../blob1.tex |
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\section{Introduction} |
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[Outline for intro] |
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\begin{itemize} |
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\item Starting point: TQFTs via fields and local relations. |
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This gives a satisfactory treatment for semisimple TQFTs |
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(i.e.\ TQFTs for which the cylinder 1-category associated to an |
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$n{-}1$-manifold $Y$ is semisimple for all $Y$). |
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\item For non-semiemple TQFTs, this approach is less satisfactory. |
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Our main motivating example (though we will not develop it in this paper) |
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is the $4{+}1$-dimensional TQFT associated to Khovanov homology. |
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It associates a bigraded vector space $A_{Kh}(W^4, L)$ to a 4-manifold $W$ together |
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with a link $L \subset \bd W$. |
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The original Khovanov homology of a link in $S^3$ is recovered as $A_{Kh}(B^4, L)$. |
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\item How would we go about computing $A_{Kh}(W^4, L)$? |
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For $A_{Kh}(B^4, L)$, the main tool is the exact triangle (long exact sequence) |
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\nn{... $L_1, L_2, L_3$}. |
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Unfortunately, the exactness breaks if we glue $B^4$ to itself and attempt |
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to compute $A_{Kh}(S^1\times B^3, L)$. |
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According to the gluing theorem for TQFTs-via-fields, gluing along $B^3 \subset \bd B^4$ |
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corresponds to taking a coend (self tensor product) over the cylinder category |
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associated to $B^3$ (with appropriate boundary conditions). |
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The coend is not an exact functor, so the exactness of the triangle breaks. |
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\item The obvious solution to this problem is to replace the coend with its derived counterpart. |
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This presumably works fine for $S^1\times B^3$ (the answer being the Hochschild homology |
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of an appropriate bimodule), but for more complicated 4-manifolds this leaves much to be desired. |
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If we build our manifold up via a handle decomposition, the computation |
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would be a sequence of derived coends. |
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A different handle decomposition of the same manifold would yield a different |
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sequence of derived coends. |
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To show that our definition in terms of derived coends is well-defined, we |
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would need to show that the above two sequences of derived coends yield the same answer. |
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This is probably not easy to do. |
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\item Instead, we would prefer a definition for a derived version of $A_{Kh}(W^4, L)$ |
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which is manifestly invariant. |
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(That is, a definition that does not |
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involve choosing a decomposition of $W$. |
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After all, one of the virtues of our starting point --- TQFTs via field and local relations --- |
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is that it has just this sort of manifest invariance.) |
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\item The solution is to replace $A_{Kh}(W^4, L)$, which is a quotient |
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\[ |
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\text{linear combinations of fields} \;\big/\; \text{local relations} , |
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\] |
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with an appropriately free resolution (the ``blob complex") |
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\[ |
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\cdots\to \bc_2(W, L) \to \bc_1(W, L) \to \bc_0(W, L) . |
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\] |
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Here $\bc_0$ is linear combinations of fields on $W$, |
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$\bc_1$ is linear combinations of local relations on $W$, |
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$\bc_2$ is linear combinations of relations amongst relations on $W$, |
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and so on. |
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\item None of the above ideas depend on the details of the Khovanov homology example, |
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so we develop the general theory in the paper and postpone specific applications |
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to later papers. |
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\item The blob complex enjoys the following nice properties \nn{...} |
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\end{itemize} |
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\bigskip |
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\hrule |
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\bigskip |
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We then show that blob homology enjoys the following |
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\ref{property:gluing} properties. |
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\begin{property}[Functoriality] |
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\label{property:functoriality}% |
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Blob homology is functorial with respect to diffeomorphisms. That is, fixing an $n$-dimensional system of fields $\cF$ and local relations $\cU$, the association |
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\begin{equation*} |
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X \mapsto \bc_*^{\cF,\cU}(X) |
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\end{equation*} |
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is a functor from $n$-manifolds and diffeomorphisms between them to chain complexes and isomorphisms between them. |
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\end{property} |
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\begin{property}[Disjoint union] |
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\label{property:disjoint-union} |
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The blob complex of a disjoint union is naturally the tensor product of the blob complexes. |
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\begin{equation*} |
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\bc_*(X_1 \du X_2) \iso \bc_*(X_1) \tensor \bc_*(X_2) |
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\end{equation*} |
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\end{property} |
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\begin{property}[A map for gluing] |
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\label{property:gluing-map}% |
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If $X_1$ and $X_2$ are $n$-manifolds, with $Y$ a codimension $0$-submanifold of $\bdy X_1$, and $Y^{\text{op}}$ a codimension $0$-submanifold of $\bdy X_2$, |
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there is a chain map |
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\begin{equation*} |
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\gl_Y: \bc_*(X_1) \tensor \bc_*(X_2) \to \bc_*(X_1 \cup_Y X_2). |
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\end{equation*} |
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\end{property} |
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\begin{property}[Contractibility] |
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\label{property:contractibility}% |
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\todo{Err, requires a splitting?} |
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The blob complex for an $n$-category on an $n$-ball is quasi-isomorphic to its $0$-th homology. |
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\begin{equation} |
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\xymatrix{\bc_*^{\cC}(B^n) \ar[r]^{\iso}_{\text{qi}} & H_0(\bc_*^{\cC}(B^n))} |
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\end{equation} |
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\todo{Say that this is just the original $n$-category?} |
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\end{property} |
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\begin{property}[Skein modules] |
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\label{property:skein-modules}% |
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The $0$-th blob homology of $X$ is the usual |
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(dual) TQFT Hilbert space (a.k.a.\ skein module) associated to $X$ |
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by $(\cF,\cU)$. (See \S \ref{sec:local-relations}.) |
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\begin{equation*} |
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H_0(\bc_*^{\cF,\cU}(X)) \iso A^{\cF,\cU}(X) |
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\end{equation*} |
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\end{property} |
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\begin{property}[Hochschild homology when $X=S^1$] |
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\label{property:hochschild}% |
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The blob complex for a $1$-category $\cC$ on the circle is |
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quasi-isomorphic to the Hochschild complex. |
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\begin{equation*} |
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\xymatrix{\bc_*^{\cC}(S^1) \ar[r]^{\iso}_{\text{qi}} & HC_*(\cC)} |
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\end{equation*} |
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\end{property} |
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\begin{property}[Evaluation map] |
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\label{property:evaluation}% |
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There is an `evaluation' chain map |
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\begin{equation*} |
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\ev_X: \CD{X} \tensor \bc_*(X) \to \bc_*(X). |
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\end{equation*} |
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(Here $\CD{X}$ is the singular chain complex of the space of diffeomorphisms of $X$, fixed on $\bdy X$.) |
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Restricted to $C_0(\Diff(X))$ this is just the action of diffeomorphisms described in Property \ref{property:functoriality}. Further, for |
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any codimension $1$-submanifold $Y \subset X$ dividing $X$ into $X_1 \cup_Y X_2$, the following diagram |
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(using the gluing maps described in Property \ref{property:gluing-map}) commutes. |
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\begin{equation*} |
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\xymatrix{ |
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\CD{X} \otimes \bc_*(X) \ar[r]^{\ev_X} & \bc_*(X) \\ |
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\CD{X_1} \otimes \CD{X_2} \otimes \bc_*(X_1) \otimes \bc_*(X_2) |
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\ar@/_4ex/[r]_{\ev_{X_1} \otimes \ev_{X_2}} \ar[u]^{\gl^{\Diff}_Y \otimes \gl_Y} & |
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\bc_*(X_1) \otimes \bc_*(X_2) \ar[u]_{\gl_Y} |
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} |
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\end{equation*} |
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\nn{should probably say something about associativity here (or not?)} |
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\end{property} |
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\begin{property}[Gluing formula] |
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\label{property:gluing}% |
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\mbox{}% <-- gets the indenting right |
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\begin{itemize} |
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\item For any $(n-1)$-manifold $Y$, the blob homology of $Y \times I$ is |
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naturally an $A_\infty$ category. % We'll write $\bc_*(Y)$ for $\bc_*(Y \times I)$ below. |
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\item For any $n$-manifold $X$, with $Y$ a codimension $0$-submanifold of its boundary, the blob homology of $X$ is naturally an |
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$A_\infty$ module for $\bc_*(Y \times I)$. |
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\item For any $n$-manifold $X$, with $Y \cup Y^{\text{op}}$ a codimension |
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$0$-submanifold of its boundary, the blob homology of $X'$, obtained from |
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$X$ by gluing along $Y$, is the $A_\infty$ self-tensor product of |
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$\bc_*(X)$ as an $\bc_*(Y \times I)$-bimodule. |
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\begin{equation*} |
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\bc_*(X') \iso \bc_*(X) \Tensor^{A_\infty}_{\mathclap{\bc_*(Y \times I)}} \!\!\!\!\!\!\xymatrix{ \ar@(ru,rd)@<-1ex>[]} |
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\end{equation*} |
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\end{itemize} |
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\end{property} |
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\begin{property}[Relation to mapping spaces] |
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There is a version of the blob complex for $C$ an $A_\infty$ $n$-category |
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instead of a garden variety $n$-category. |
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Let $\pi^\infty_{\le n}(W)$ denote the $A_\infty$ $n$-category based on maps |
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$B^n \to W$. |
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(The case $n=1$ is the usual $A_\infty$ category of paths in $W$.) |
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Then $\bc_*(M, \pi^\infty_{\le n}(W))$ is |
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homotopy equivalent to $C_*(\{\text{maps}\; M \to W\})$. |
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\end{property} |
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\begin{property}[Product formula] |
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Let $M^n = Y^{n-k}\times W^k$ and let $C$ be an $n$-category. |
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Let $A_*(Y)$ be the $A_\infty$ $k$-category associated to $Y$ via blob homology. |
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Then |
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\[ |
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\bc_*(Y^{n-k}\times W^k, C) \simeq \bc_*(W, A_*(Y)) . |
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\] |
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\nn{say something about general fiber bundles?} |
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\end{property} |
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\begin{property}[Higher dimensional Deligne conjecture] |
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The singular chains of the $n$-dimensional fat graph operad act on blob cochains. |
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The $n$-dimensional fat graph operad can be thought of as a sequence of general surgeries |
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of $n$-manifolds |
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$R_i \cup A_i \leadsto R_i \cup B_i$ together with mapping cylinders of diffeomorphisms |
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$f_i: R_i\cup B_i \to R_{i+1}\cup A_{i+1}$. |
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(Note that the suboperad where $A_i$, $B_i$ and $R_i\cup A_i$ are all diffeomorphic to |
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the $n$-ball is equivalent to the little $n{+}1$-disks operad.) |
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If $A$ and $B$ are $n$-manifolds sharing the same boundary, define |
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the blob cochains $\bc^*(A, B)$ (analogous to Hochschild cohomology) to be |
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$A_\infty$ maps from $\bc_*(A)$ to $\bc_*(B)$, where we think of both |
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(collections of) complexes as modules over the $A_\infty$ category associated to $\bd A = \bd B$. |
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The ``holes" in the above |
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$n$-dimensional fat graph operad are labeled by $\bc^*(A_i, B_i)$. |
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\end{property} |
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Properties \ref{property:functoriality}, \ref{property:gluing-map} and \ref{property:skein-modules} will be immediate from the definition given in |
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\S \ref{sec:blob-definition}, and we'll recall them at the appropriate points there. \todo{Make sure this gets done.} |
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Properties \ref{property:disjoint-union} and \ref{property:contractibility} are established in \S \ref{sec:basic-properties}. |
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Property \ref{property:hochschild} is established in \S \ref{sec:hochschild}, Property \ref{property:evaluation} in \S \ref{sec:evaluation}, |
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and Property \ref{property:gluing} in \S \ref{sec:gluing}. |
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\nn{need to say where the remaining properties are proved.} |