text/ncat.tex
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   659 In addition, collar maps act trivially on $\cC(X)$.
   659 In addition, collar maps act trivially on $\cC(X)$.
   660 \end{axiom}
   660 \end{axiom}
   661 
   661 
   662 \medskip
   662 \medskip
   663 
   663 
   664 
   664 This completes the definition of an $n$-category.
   665 
   665 Next we define enriched $n$-categories.
   666 
   666 
   667 \nn{begin temp relocation}
   667 \medskip
       
   668 
   668 
   669 
   669 Most of the examples of $n$-categories we are interested in are enriched in the following sense.
   670 Most of the examples of $n$-categories we are interested in are enriched in the following sense.
   670 The various sets of $n$-morphisms $\cC(X; c)$, for all $n$-balls $X$ and
   671 The various sets of $n$-morphisms $\cC(X; c)$, for all $n$-balls $X$ and
   671 all $c\in \cl{\cC}(\bd X)$, have the structure of an object in some auxiliary symmetric monoidal category
   672 all $c\in \cl{\cC}(\bd X)$, have the structure of an object in some appropriate auxiliary category
   672 with sufficient limits and colimits
       
   673 (e.g.\ vector spaces, or modules over some ring, or chain complexes),
   673 (e.g.\ vector spaces, or modules over some ring, or chain complexes),
   674 %\nn{actually, need both disj-union/sum and product/tensor-product; what's the name for this sort of cat?}
   674 and all the structure maps of the $n$-category are compatible with the auxiliary
   675 and all the structure maps of the $n$-category should be compatible with the auxiliary
       
   676 category structure.
   675 category structure.
   677 Note that this auxiliary structure is only in dimension $n$; if $\dim(Y) < n$ then 
   676 Note that this auxiliary structure is only in dimension $n$; if $\dim(Y) < n$ then 
   678 $\cC(Y; c)$ is just a plain set.
   677 $\cC(Y; c)$ is just a plain set.
   679 
   678 
       
   679 We will aim for a little bit more generality than we need and not assume that the objects
       
   680 of our auxiliary category are sets with extra structure.
       
   681 First we must specify requirements for the auxiliary category.
       
   682 It should have a {\it distributive monoidal structure} in the sense of 
       
   683 \nn{Stolz and Teichner, Traces in monoidal categories, 1010.4527}.
       
   684 This means that there is a monoidal structure $\otimes$ and also coproduct $\oplus$,
       
   685 and these two structures interact in the appropriate way.
       
   686 Examples include 
       
   687 \begin{itemize}
       
   688 \item vector spaces (or $R$-modules or chain complexes) with tensor product and direct sum; and
       
   689 \item topological spaces with product and disjoint union.
       
   690 \end{itemize}
       
   691 Before stating the revised axioms for an $n$-category enriched in a distributive monoidal category,
       
   692 we need a preliminary definition.
       
   693 Once we have the above $n$-category axioms for $n{-}1$-morphisms, we can define the 
       
   694 category $\bbc$ of {\it $n$-balls with boundary conditions}.
       
   695 Its objects are pairs $(X, c)$, where $X$ is an $n$-ball and $c \in \cl\cC(\bd X)$ is the ``boundary condition".
       
   696 Its morphisms are homeomorphisms $f:X\to X$ such that $f|_{\bd X}(c) = c$.
       
   697  
       
   698 \begin{axiom}[Enriched $n$-categories]
       
   699 \label{axiom:enriched}
       
   700 Let $\cS$ be a distributive symmetric monoidal category.
       
   701 An $n$-category enriched in $\cS$ satisfies the above $n$-category axioms for $k=0,\ldots,n-1$,
       
   702 and modifies the axioms for $k=n$ as follows:
       
   703 \begin{itemize}
       
   704 \item Morphisms. We have a functor $\cC_n$ from $\bbc$ ($n$-balls with boundary conditions) to $\cS$.
       
   705 \item Composition. Let $B = B_1\cup_Y B_2$ as in Axiom \ref{axiom:composition}.
       
   706 Let $Y_i = \bd B_i \setmin Y$.  
       
   707 Note that $\bd B = Y_1\cup Y_2$.
       
   708 Let $c_i \in \cC(Y_i)$ with $\bd c_1 = \bd c_2 = d \in \cl\cC(E)$.
       
   709 Then we have a map
       
   710 \[
       
   711 	\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),
       
   712 \]
       
   713 where the sum is over $c\in\cC(Y)$ such that $\bd c = d$.
       
   714 This map is natural with respect to the action of homeomorphisms and with respect to restrictions.
       
   715 \item Product morphisms. \nn{Hmm... not sure what to say here. maybe we need sets with structure after all.}
       
   716 \end{itemize}
       
   717 \end{axiom}
       
   718 
       
   719 
       
   720 
       
   721 
       
   722 \nn{a-inf starts by enriching over inf-cat; maybe simple version first, then peter's version (attrib to peter)}
       
   723 
       
   724 \nn{blarg}
       
   725 
   680 \nn{$k=n$ injectivity for a-inf (necessary?)}
   726 \nn{$k=n$ injectivity for a-inf (necessary?)}
   681 or if $k=n$ and we are in the $A_\infty$ case, 
   727 or if $k=n$ and we are in the $A_\infty$ case, 
   682 
   728 
   683 
   729 
   684 \nn{end temp relocation}
   730 \nn{resume revising here}
   685 
   731 
   686 
   732 
   687 \smallskip
   733 \smallskip
   688 
   734 
   689 For $A_\infty$ $n$-categories, we replace
   735 For $A_\infty$ $n$-categories, we replace
   735 $\cC(X)$ is a trivial 1-element set if $X$ is a $k$-ball with $k<j$.
   781 $\cC(X)$ is a trivial 1-element set if $X$ is a $k$-ball with $k<j$.
   736 See Example \ref{ex:bord-cat}.
   782 See Example \ref{ex:bord-cat}.
   737 
   783 
   738 \medskip
   784 \medskip
   739 
   785 
   740 The alert reader will have already noticed that our definition of a (ordinary) $n$-category
   786 The alert reader will have already noticed that our definition of an (ordinary) $n$-category
   741 is extremely similar to our definition of a system of fields.
   787 is extremely similar to our definition of a system of fields.
   742 There are two differences.
   788 There are two differences.
   743 First, for the $n$-category definition we restrict our attention to balls
   789 First, for the $n$-category definition we restrict our attention to balls
   744 (and their boundaries), while for fields we consider all manifolds.
   790 (and their boundaries), while for fields we consider all manifolds.
   745 Second,  in category definition we directly impose isotopy
   791 Second,  in category definition we directly impose isotopy