text/appendixes/famodiff.tex
changeset 794 24f14faacab4
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   233 \begin{proof}
   233 \begin{proof}
   234 We will imitate the proof of Corollary 1.3 of \cite{MR0283802}.
   234 We will imitate the proof of Corollary 1.3 of \cite{MR0283802}.
   235 
   235 
   236 Let $P$ be some $k$-dimensional polyhedron and $f:P\to \Homeo(X)$.
   236 Let $P$ be some $k$-dimensional polyhedron and $f:P\to \Homeo(X)$.
   237 After subdividing $P$, we may assume that there exists $g\in \Homeo(X)$
   237 After subdividing $P$, we may assume that there exists $g\in \Homeo(X)$
   238 such that $g\circ f(P)$ is a small neighborhood of the 
   238 such that $g^{-1}\circ f(P)$ is a small neighborhood of the 
   239 identity in $\Homeo(X)$.
   239 identity in $\Homeo(X)$.
   240 The sense of ``small" we mean will be explained below.
   240 The sense of ``small" we mean will be explained below.
   241 It depends only on $\cU$ and some auxiliary covers.
   241 It depends only on $\cU$ and some auxiliary covers.
   242 
   242 
   243 We assume, inductively, that the restriction of $f$ to $\bd P$ is adapted to $\cU$.
   243 We may assume, inductively, that the restriction of $f$ to $\bd P$ is adapted to $\cU$.
   244 
   244 
   245 
   245 Since $X$ is compact, we may assume without loss of generality that the cover $\cU$ is finite.
   246 
   246 Let $\cU = \{U_\alpha\}$, $1\le \alpha\le N$.
   247 \nn{...}
   247 
   248 
   248 We will need some wiggle room, so for each $\alpha$ choose open sets
       
   249 \[
       
   250 	U_\alpha = U_\alpha^0 \supset U_\alpha^1 \supset \cdots \supset U_\alpha^N
       
   251 \]
       
   252 so that for each fixed $i$ the sets $\{U_\alpha^i\}$ are an open cover of $X$, and also so that
       
   253 the closure $\ol{U_\alpha^i}$ is compact and $U_\alpha^{i-1} \supset \ol{U_\alpha^i}$.
       
   254 
       
   255 Theorem 5.1 of \cite{MR0283802}, applied $N$ times, allows us
       
   256 to choose a homotopy $h:P\times [0,N] \to \Homeo(X)$ with the following properties:
       
   257 \begin{itemize}
       
   258 \item $h(p, 0) = f(p)$ for all $p\in P$.
       
   259 \item The restriction of $h(p, i)$ to $U_0^i \cup \cdots \cup U_i^i$ is equal to the homeomorphism $g$,
       
   260 for all $p\in P$.
       
   261 \item For each fixed $p\in P$, the family of homeomorphisms $h(p, [i-1, i])$ is supported on $U_i^{i-1}$
       
   262 (and hence supported on $U_i$).
       
   263 \end{itemize}
       
   264 To apply Theorem 5.1 of \cite{MR0283802}, the family $f(P)$ must be sufficiently small,
       
   265 and the subdivision mentioned above is chosen fine enough to insure this.
       
   266 
       
   267 By reparametrizing, we can view $h$ as a homotopy (rel boundary) from $h(\cdot,0) = f: P\to\Homeo(X)$
       
   268 to the family
       
   269 \[
       
   270 	h(\bd(P\times [0,N]) \setmin P\times \{0\}) = h(\bd P \times [0,N]) \cup h(P \times \{N\}) .
       
   271 \]
       
   272 We claim that the latter family of homeomorphisms is adapted to $\cU$.
       
   273 By the second bullet above, $h(P\times \{N\})$ is the constant family $g$, and is therefore supported on the empty set.
       
   274 Via an inductive assumption and a preliminary homotopy, we have arranged above that $f(\bd P)$ is
       
   275 adapted to $\cU$, which means that $\bd P = \cup_j Q_j$ with $f(Q_j)$ supported on the union of $k-1$
       
   276 of the $U_\alpha$'s for each $j$.
       
   277 It follows (using the third bullet above) that $h(Q_j \times [i-1,i])$ is supported on the union of $k$ 
       
   278 of the $U_\alpha$'s, specifically, the support of $f(Q_j)$ union $U_i$.
   249 \end{proof}
   279 \end{proof}
   250 
   280 
   251 
   281 
   252 
   282 
   253 \begin{lemma} \label{extension_lemma_c}
   283 \begin{lemma} \label{extension_lemma_c}