diff -r 9698f584e732 -r f7da004e1f14 text/tqftreview.tex --- a/text/tqftreview.tex Fri Jun 04 08:15:08 2010 -0700 +++ b/text/tqftreview.tex Fri Jun 04 11:42:07 2010 -0700 @@ -5,7 +5,15 @@ \label{sec:tqftsviafields} In this section we review the notion of a ``system of fields and local relations". -For more details see \cite{kw:tqft}. From a system of fields and local relations we can readily construct TQFT invariants of manifolds. This is described in \S \ref{sec:constructing-a-tqft}. A system of fields is very closely related to an $n$-category. In Example \ref{ex:traditional-n-categories(fields)}, which runs throughout this section, we sketch the construction of a system of fields from an $n$-category. We make this more precise for $n=1$ or $2$ in \S \ref{sec:example:traditional-n-categories(fields)}, and much later, after we've have given our own definition of a `topological $n$-category' in \S \ref{sec:ncats}, we explain precisely how to go back and forth between a topological $n$-category and a system of fields and local relations. +For more details see \cite{kw:tqft}. +From a system of fields and local relations we can readily construct TQFT invariants of manifolds. +This is described in \S \ref{sec:constructing-a-tqft}. +A system of fields is very closely related to an $n$-category. +In Example \ref{ex:traditional-n-categories(fields)}, which runs throughout this section, +we sketch the construction of a system of fields from an $n$-category. +We make this more precise for $n=1$ or $2$ in \S \ref{sec:example:traditional-n-categories(fields)}, +and much later, after we've have given our own definition of a `topological $n$-category' in \S \ref{sec:ncats}, +we explain precisely how to go back and forth between a topological $n$-category and a system of fields and local relations. We only consider compact manifolds, so if $Y \sub X$ is a closed codimension 0 submanifold of $X$, then $X \setmin Y$ implicitly means the closure @@ -21,7 +29,9 @@ oriented, topological, smooth, spin, etc. --- but for definiteness we will stick with unoriented PL.) -Fix a symmetric monoidal category $\cS$. While reading the definition, you should just think about the cases $\cS = \Set$ or $\cS = \Vect$. The presentation here requires that the objects of $\cS$ have an underlying set, but this could probably be avoided if desired. +Fix a symmetric monoidal category $\cS$. +While reading the definition, you should just think about the cases $\cS = \Set$ or $\cS = \Vect$. +The presentation here requires that the objects of $\cS$ have an underlying set, but this could probably be avoided if desired. A $n$-dimensional {\it system of fields} in $\cS$ is a collection of functors $\cC_k : \cM_k \to \Set$ for $0 \leq k \leq n$ @@ -54,7 +64,10 @@ For $c \in \cC_{k-1}(\bd X)$, we will denote by $\cC_k(X; c)$ the subset of $\cC(X)$ which restricts to $c$. In this context, we will call $c$ a boundary condition. -\item The subset $\cC_n(X;c)$ of top fields with a given boundary condition is an object in our symmetric monoidal category $\cS$. (This condition is of course trivial when $\cS = \Set$.) If the objects are sets with extra structure (e.g. $\cS = \Vect$ or $\Kom$), then this extra structure is considered part of the definition of $\cC_n$. Any maps mentioned below between top level fields must be morphisms in $\cS$. +\item The subset $\cC_n(X;c)$ of top fields with a given boundary condition is an object in our symmetric monoidal category $\cS$. +(This condition is of course trivial when $\cS = \Set$.) If the objects are sets with extra structure (e.g. $\cS = \Vect$ or $\Kom$), +then this extra structure is considered part of the definition of $\cC_n$. +Any maps mentioned below between top level fields must be morphisms in $\cS$. \item $\cC_k$ is compatible with the symmetric monoidal structures on $\cM_k$, $\Set$ and $\cS$: $\cC_k(X \du W) \cong \cC_k(X)\times \cC_k(W)$, compatibly with homeomorphisms and restriction to boundary. @@ -185,11 +198,12 @@ \subsection{Systems of fields from $n$-categories} \label{sec:example:traditional-n-categories(fields)} -We now describe in more detail Example \ref{ex:traditional-n-categories(fields)}, systems of fields coming from sub-cell-complexes labeled +We now describe in more detail Example \ref{ex:traditional-n-categories(fields)}, +systems of fields coming from sub-cell-complexes labeled by $n$-category morphisms. Given an $n$-category $C$ with the right sort of duality -(e.g. a pivotal 2-category, 1-category with duals, star 1-category), +(e.g. a pivotal 2-category, *-1-category), we can construct a system of fields as follows. Roughly speaking, $\cC(X)$ will the set of all embedded cell complexes in $X$ with codimension $i$ cells labeled by $i$-morphisms of $C$. @@ -197,7 +211,8 @@ If $X$ has boundary, we require that the cell decompositions are in general position with respect to the boundary --- the boundary intersects each cell -transversely, so cells meeting the boundary are mere half-cells. Put another way, the cell decompositions we consider are dual to standard cell +transversely, so cells meeting the boundary are mere half-cells. +Put another way, the cell decompositions we consider are dual to standard cell decompositions of $X$. We will always assume that our $n$-categories have linear $n$-morphisms. @@ -270,7 +285,8 @@ \subsection{Local relations} \label{sec:local-relations} -Local relations are certain subspaces of the fields on balls, which form an ideal under gluing. Again, we give the examples first. +Local relations are certain subspaces of the fields on balls, which form an ideal under gluing. +Again, we give the examples first. \addtocounter{prop}{-2} \begin{example}[contd.] @@ -353,7 +369,8 @@ Let $Y$ be an $n{-}1$-manifold. Define a (linear) 1-category $A(Y)$ as follows. The objects of $A(Y)$ are $\cC(Y)$. -The morphisms from $a$ to $b$ are $A(Y\times I; a, b)$, where $a$ and $b$ label the two boundary components of the cylinder $Y\times I$. +The morphisms from $a$ to $b$ are $A(Y\times I; a, b)$, +where $a$ and $b$ label the two boundary components of the cylinder $Y\times I$. Composition is given by gluing of cylinders. Let $X$ be an $n$-manifold with boundary and consider the collection of vector spaces