text/tqftreview.tex
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     3 \section{TQFTs via fields}
     3 \section{TQFTs via fields}
     4 \label{sec:fields}
     4 \label{sec:fields}
     5 \label{sec:tqftsviafields}
     5 \label{sec:tqftsviafields}
     6 
     6 
     7 In this section we review the notion of a ``system of fields and local relations".
     7 In this section we review the notion of a ``system of fields and local relations".
     8 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.
     8 For more details see \cite{kw:tqft}.
       
     9 From a system of fields and local relations we can readily construct TQFT invariants of manifolds.
       
    10 This is described in \S \ref{sec:constructing-a-tqft}.
       
    11 A system of fields is very closely related to an $n$-category.
       
    12 In Example \ref{ex:traditional-n-categories(fields)}, which runs throughout this section, 
       
    13 we sketch the construction of a system of fields from an $n$-category.
       
    14 We make this more precise for $n=1$ or $2$ in \S \ref{sec:example:traditional-n-categories(fields)}, 
       
    15 and much later, after we've have given our own definition of a `topological $n$-category' in \S \ref{sec:ncats}, 
       
    16 we explain precisely how to go back and forth between a topological $n$-category and a system of fields and local relations.
     9 
    17 
    10 We only consider compact manifolds, so if $Y \sub X$ is a closed codimension 0
    18 We only consider compact manifolds, so if $Y \sub X$ is a closed codimension 0
    11 submanifold of $X$, then $X \setmin Y$ implicitly means the closure
    19 submanifold of $X$, then $X \setmin Y$ implicitly means the closure
    12 $\overline{X \setmin Y}$.
    20 $\overline{X \setmin Y}$.
    13 
    21 
    19 $k$ and morphisms homeomorphisms.
    27 $k$ and morphisms homeomorphisms.
    20 (We could equally well work with a different category of manifolds ---
    28 (We could equally well work with a different category of manifolds ---
    21 oriented, topological, smooth, spin, etc. --- but for definiteness we
    29 oriented, topological, smooth, spin, etc. --- but for definiteness we
    22 will stick with unoriented PL.)
    30 will stick with unoriented PL.)
    23 
    31 
    24 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.
    32 Fix a symmetric monoidal category $\cS$.
       
    33 While reading the definition, you should just think about the cases $\cS = \Set$ or $\cS = \Vect$.
       
    34 The presentation here requires that the objects of $\cS$ have an underlying set, but this could probably be avoided if desired.
    25 
    35 
    26 A $n$-dimensional {\it system of fields} in $\cS$
    36 A $n$-dimensional {\it system of fields} in $\cS$
    27 is a collection of functors $\cC_k : \cM_k \to \Set$ for $0 \leq k \leq n$
    37 is a collection of functors $\cC_k : \cM_k \to \Set$ for $0 \leq k \leq n$
    28 together with some additional data and satisfying some additional conditions, all specified below.
    38 together with some additional data and satisfying some additional conditions, all specified below.
    29 
    39 
    52 and these maps comprise a natural
    62 and these maps comprise a natural
    53 transformation between the functors $\cC_k$ and $\cC_{k-1}\circ\bd$.
    63 transformation between the functors $\cC_k$ and $\cC_{k-1}\circ\bd$.
    54 For $c \in \cC_{k-1}(\bd X)$, we will denote by $\cC_k(X; c)$ the subset of 
    64 For $c \in \cC_{k-1}(\bd X)$, we will denote by $\cC_k(X; c)$ the subset of 
    55 $\cC(X)$ which restricts to $c$.
    65 $\cC(X)$ which restricts to $c$.
    56 In this context, we will call $c$ a boundary condition.
    66 In this context, we will call $c$ a boundary condition.
    57 \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 $\cS$.
    67 \item The subset $\cC_n(X;c)$ of top fields with a given boundary condition is an object in our symmetric monoidal category $\cS$.
       
    68 (This condition is of course trivial when $\cS = \Set$.) If the objects are sets with extra structure (e.g. $\cS = \Vect$ or $\Kom$), 
       
    69 then this extra structure is considered part of the definition of $\cC_n$.
       
    70 Any maps mentioned below between top level fields must be morphisms in $\cS$.
    58 \item $\cC_k$ is compatible with the symmetric monoidal
    71 \item $\cC_k$ is compatible with the symmetric monoidal
    59 structures on $\cM_k$, $\Set$ and $\cS$: $\cC_k(X \du W) \cong \cC_k(X)\times \cC_k(W)$,
    72 structures on $\cM_k$, $\Set$ and $\cS$: $\cC_k(X \du W) \cong \cC_k(X)\times \cC_k(W)$,
    60 compatibly with homeomorphisms and restriction to boundary.
    73 compatibly with homeomorphisms and restriction to boundary.
    61 We will call the projections $\cC(X_1 \du X_2) \to \cC(X_i)$
    74 We will call the projections $\cC(X_1 \du X_2) \to \cC(X_i)$
    62 restriction maps.
    75 restriction maps.
   183 above tensor products.
   196 above tensor products.
   184 
   197 
   185 
   198 
   186 \subsection{Systems of fields from $n$-categories}
   199 \subsection{Systems of fields from $n$-categories}
   187 \label{sec:example:traditional-n-categories(fields)}
   200 \label{sec:example:traditional-n-categories(fields)}
   188 We now describe in more detail Example \ref{ex:traditional-n-categories(fields)}, systems of fields coming from sub-cell-complexes labeled
   201 We now describe in more detail Example \ref{ex:traditional-n-categories(fields)}, 
       
   202 systems of fields coming from sub-cell-complexes labeled
   189 by $n$-category morphisms.
   203 by $n$-category morphisms.
   190 
   204 
   191 Given an $n$-category $C$ with the right sort of duality
   205 Given an $n$-category $C$ with the right sort of duality
   192 (e.g. a pivotal 2-category, 1-category with duals, star 1-category),
   206 (e.g. a pivotal 2-category, *-1-category),
   193 we can construct a system of fields as follows.
   207 we can construct a system of fields as follows.
   194 Roughly speaking, $\cC(X)$ will the set of all embedded cell complexes in $X$
   208 Roughly speaking, $\cC(X)$ will the set of all embedded cell complexes in $X$
   195 with codimension $i$ cells labeled by $i$-morphisms of $C$.
   209 with codimension $i$ cells labeled by $i$-morphisms of $C$.
   196 We'll spell this out for $n=1,2$ and then describe the general case.
   210 We'll spell this out for $n=1,2$ and then describe the general case.
   197 
   211 
   198 If $X$ has boundary, we require that the cell decompositions are in general
   212 If $X$ has boundary, we require that the cell decompositions are in general
   199 position with respect to the boundary --- the boundary intersects each cell
   213 position with respect to the boundary --- the boundary intersects each cell
   200 transversely, so cells meeting the boundary are mere half-cells. Put another way, the cell decompositions we consider are dual to standard cell
   214 transversely, so cells meeting the boundary are mere half-cells.
       
   215 Put another way, the cell decompositions we consider are dual to standard cell
   201 decompositions of $X$.
   216 decompositions of $X$.
   202 
   217 
   203 We will always assume that our $n$-categories have linear $n$-morphisms.
   218 We will always assume that our $n$-categories have linear $n$-morphisms.
   204 
   219 
   205 For $n=1$, a field on a 0-manifold $P$ is a labeling of each point of $P$ with
   220 For $n=1$, a field on a 0-manifold $P$ is a labeling of each point of $P$ with
   268 
   283 
   269 
   284 
   270 
   285 
   271 \subsection{Local relations}
   286 \subsection{Local relations}
   272 \label{sec:local-relations}
   287 \label{sec:local-relations}
   273 Local relations are certain subspaces of the fields on balls, which form an ideal under gluing. Again, we give the examples first.
   288 Local relations are certain subspaces of the fields on balls, which form an ideal under gluing.
       
   289 Again, we give the examples first.
   274 
   290 
   275 \addtocounter{prop}{-2}
   291 \addtocounter{prop}{-2}
   276 \begin{example}[contd.]
   292 \begin{example}[contd.]
   277 For maps into spaces, $U(B; c)$ is generated by fields of the form $a-b \in \lf(B; c)$,
   293 For maps into spaces, $U(B; c)$ is generated by fields of the form $a-b \in \lf(B; c)$,
   278 where $a$ and $b$ are maps (fields) which are homotopic rel boundary.
   294 where $a$ and $b$ are maps (fields) which are homotopic rel boundary.
   351 that lacks its $n{+}1$-dimensional part.)
   367 that lacks its $n{+}1$-dimensional part.)
   352 
   368 
   353 Let $Y$ be an $n{-}1$-manifold.
   369 Let $Y$ be an $n{-}1$-manifold.
   354 Define a (linear) 1-category $A(Y)$ as follows.
   370 Define a (linear) 1-category $A(Y)$ as follows.
   355 The objects of $A(Y)$ are $\cC(Y)$.
   371 The objects of $A(Y)$ are $\cC(Y)$.
   356 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 $Y\times I$.
   372 The morphisms from $a$ to $b$ are $A(Y\times I; a, b)$, 
       
   373 where $a$ and $b$ label the two boundary components of the cylinder $Y\times I$.
   357 Composition is given by gluing of cylinders.
   374 Composition is given by gluing of cylinders.
   358 
   375 
   359 Let $X$ be an $n$-manifold with boundary and consider the collection of vector spaces
   376 Let $X$ be an $n$-manifold with boundary and consider the collection of vector spaces
   360 $A(X; \cdot) \deq \{A(X; c)\}$ where $c$ ranges through $\cC(\bd X)$.
   377 $A(X; \cdot) \deq \{A(X; c)\}$ where $c$ ranges through $\cC(\bd X)$.
   361 This collection of vector spaces affords a representation of the category $A(\bd X)$, where
   378 This collection of vector spaces affords a representation of the category $A(\bd X)$, where