text/ncat.tex
changeset 207 d5cbbc87c340
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  1170 circles (1-spheres) which we just introduced.
  1170 circles (1-spheres) which we just introduced.
  1171 
  1171 
  1172 Equivalently, we can define 1-sphere modules in terms of 1-marked $k$-balls, $2\le k\le n$.
  1172 Equivalently, we can define 1-sphere modules in terms of 1-marked $k$-balls, $2\le k\le n$.
  1173 Fix a marked (and labeled) circle $S$.
  1173 Fix a marked (and labeled) circle $S$.
  1174 Let $C(S)$ denote the cone of $S$, a marked 2-ball (Figure xxxx).
  1174 Let $C(S)$ denote the cone of $S$, a marked 2-ball (Figure xxxx).
  1175 \nn{I need to make up my mind whether marked things are always labeled too.}
  1175 \nn{I need to make up my mind whether marked things are always labeled too.
  1176 A 1-marked $k$-ball is anything homeomorphic to $B^j \times C(S)$, $0\le j\le n-2$.
  1176 For the time being, let's say they are.}
       
  1177 A 1-marked $k$-ball is anything homeomorphic to $B^j \times C(S)$, $0\le j\le n-2$, 
       
  1178 where $B^j$ is the standard $j$-ball.
       
  1179 1-marked $k$-balls can be decomposed in various ways into smaller balls, which are either 
       
  1180 smaller 1-marked $k$-balls or the product of an unmarked ball with a marked interval.
       
  1181 We now proceed as in the above module definitions.
       
  1182 
       
  1183 A $n$-category 1-sphere module is, among other things, an $n{-}2$-category $\cD$ with
       
  1184 \[
       
  1185 	\cD(X) \deq \cM(X\times C(S)) .
       
  1186 \]
       
  1187 The product is pinched over the boundary of $C(S)$.
       
  1188 $\cD$ breaks into ``blocks" according to the restriction to the 
       
  1189 image of $\bd C(S) = S$ in $X\times C(S)$.
       
  1190 
       
  1191 More generally, consider a 2-manifold $Y$ 
       
  1192 (e.g.\ 2-ball or 2-sphere) marked by an embedded 1-complex $K$.
       
  1193 The components of $Y\setminus K$ are labeled by $n$-categories, 
       
  1194 the edges of $K$ are labeled by 0-sphere modules, 
       
  1195 and the 0-cells of $K$ are labeled by 1-sphere modules.
       
  1196 We can now apply the coend construction and obtain an $n{-}2$-category.
       
  1197 If $Y$ has boundary then this $n{-}2$-category is a module for the $n{-}1$-manifold
       
  1198 associated to the (marked, labeled) boundary of $Y$.
       
  1199 In particular, if $\bd Y$ is a 1-sphere then we get a 1-sphere module as defined above.
       
  1200 
       
  1201 \medskip
       
  1202 
       
  1203 It should now be clear how to define $n$-category $m$-sphere modules for $0\le m \le n-1$.
       
  1204 For example, there is an $n{-}2$-category associated to a marked, labeled 2-sphere,
       
  1205 and an $m$-sphere module is a representation of such an $n{-}2$-category.
       
  1206 
       
  1207 \medskip
       
  1208 
       
  1209 We can now define the $n$- or less dimensional part of our $n{+}1$-category $\cS$.
       
  1210 Choose some collection of $n$-categories, then choose some collections of bimodules for
       
  1211 these $n$-categories, then choose some collection of 1-sphere modules for the various
       
  1212 possible marked 1-spheres labeled by the $n$-categories and bimodules, and so on.
       
  1213 Let $L_i$ denote the collection of $i{-}1$-sphere modules we have chosen.
       
  1214 (For convenience, we declare a $(-1)$-sphere module to be an $n$-category.)
       
  1215 There is a wide range of possibilities.
       
  1216 $L_0$ could contain infinitely many $n$-categories or just one.
       
  1217 For each pair of $n$-categories in $L_0$, $L_1$ could contain no bimodules at all or 
       
  1218 it could contain several.
       
  1219 
       
  1220 \nn{...}
  1177 
  1221 
  1178 \medskip
  1222 \medskip
  1179 \hrule
  1223 \hrule
  1180 \medskip
  1224 \medskip
  1181 
  1225 
  1185 
  1229 
  1186 Stuff that remains to be done (either below or in an appendix or in a separate section or in
  1230 Stuff that remains to be done (either below or in an appendix or in a separate section or in
  1187 a separate paper):
  1231 a separate paper):
  1188 \begin{itemize}
  1232 \begin{itemize}
  1189 \item spell out what difference (if any) Top vs PL vs Smooth makes
  1233 \item spell out what difference (if any) Top vs PL vs Smooth makes
  1190 \item define $n{+}1$-cat of $n$-cats (a.k.a.\ $n{+}1$-category of generalized bimodules
  1234 \item discuss Morita equivalence
  1191 a.k.a.\ $n{+}1$-category of sphere modules); discuss Morita equivalence
       
  1192 \item morphisms of modules; show that it's adjoint to tensor product
  1235 \item morphisms of modules; show that it's adjoint to tensor product
  1193 (need to define dual module for this)
  1236 (need to define dual module for this)
  1194 \item functors
  1237 \item functors
  1195 \end{itemize}
  1238 \end{itemize}
  1196 
  1239