diff -r 6a1b6c2de201 -r 787914e9e859 text/ncat.tex --- a/text/ncat.tex Fri May 27 21:54:22 2011 -0600 +++ b/text/ncat.tex Sat May 28 09:49:30 2011 -0600 @@ -661,27 +661,73 @@ \medskip - +This completes the definition of an $n$-category. +Next we define enriched $n$-categories. +\medskip -\nn{begin temp relocation} Most of the examples of $n$-categories we are interested in are enriched in the following sense. The various sets of $n$-morphisms $\cC(X; c)$, for all $n$-balls $X$ and -all $c\in \cl{\cC}(\bd X)$, have the structure of an object in some auxiliary symmetric monoidal category -with sufficient limits and colimits +all $c\in \cl{\cC}(\bd X)$, have the structure of an object in some appropriate auxiliary category (e.g.\ vector spaces, or modules over some ring, or chain complexes), -%\nn{actually, need both disj-union/sum and product/tensor-product; what's the name for this sort of cat?} -and all the structure maps of the $n$-category should be compatible with the auxiliary +and all the structure maps of the $n$-category are compatible with the auxiliary category structure. Note that this auxiliary structure is only in dimension $n$; if $\dim(Y) < n$ then $\cC(Y; c)$ is just a plain set. +We will aim for a little bit more generality than we need and not assume that the objects +of our auxiliary category are sets with extra structure. +First we must specify requirements for the auxiliary category. +It should have a {\it distributive monoidal structure} in the sense of +\nn{Stolz and Teichner, Traces in monoidal categories, 1010.4527}. +This means that there is a monoidal structure $\otimes$ and also coproduct $\oplus$, +and these two structures interact in the appropriate way. +Examples include +\begin{itemize} +\item vector spaces (or $R$-modules or chain complexes) with tensor product and direct sum; and +\item topological spaces with product and disjoint union. +\end{itemize} +Before stating the revised axioms for an $n$-category enriched in a distributive monoidal category, +we need a preliminary definition. +Once we have the above $n$-category axioms for $n{-}1$-morphisms, we can define the +category $\bbc$ of {\it $n$-balls with boundary conditions}. +Its objects are pairs $(X, c)$, where $X$ is an $n$-ball and $c \in \cl\cC(\bd X)$ is the ``boundary condition". +Its morphisms are homeomorphisms $f:X\to X$ such that $f|_{\bd X}(c) = c$. + +\begin{axiom}[Enriched $n$-categories] +\label{axiom:enriched} +Let $\cS$ be a distributive symmetric monoidal category. +An $n$-category enriched in $\cS$ satisfies the above $n$-category axioms for $k=0,\ldots,n-1$, +and modifies the axioms for $k=n$ as follows: +\begin{itemize} +\item Morphisms. We have a functor $\cC_n$ from $\bbc$ ($n$-balls with boundary conditions) to $\cS$. +\item Composition. Let $B = B_1\cup_Y B_2$ as in Axiom \ref{axiom:composition}. +Let $Y_i = \bd B_i \setmin Y$. +Note that $\bd B = Y_1\cup Y_2$. +Let $c_i \in \cC(Y_i)$ with $\bd c_1 = \bd c_2 = d \in \cl\cC(E)$. +Then we have a map +\[ + \gl_Y : \bigoplus_c \cC(B_1; c_1 \bullet c) \otimes \cC(B_2; c_2\bullet c) \to \cC(B; c_1\bullet c_2), +\] +where the sum is over $c\in\cC(Y)$ such that $\bd c = d$. +This map is natural with respect to the action of homeomorphisms and with respect to restrictions. +\item Product morphisms. \nn{Hmm... not sure what to say here. maybe we need sets with structure after all.} +\end{itemize} +\end{axiom} + + + + +\nn{a-inf starts by enriching over inf-cat; maybe simple version first, then peter's version (attrib to peter)} + +\nn{blarg} + \nn{$k=n$ injectivity for a-inf (necessary?)} or if $k=n$ and we are in the $A_\infty$ case, -\nn{end temp relocation} +\nn{resume revising here} \smallskip @@ -737,7 +783,7 @@ \medskip -The alert reader will have already noticed that our definition of a (ordinary) $n$-category +The alert reader will have already noticed that our definition of an (ordinary) $n$-category is extremely similar to our definition of a system of fields. There are two differences. First, for the $n$-category definition we restrict our attention to balls