text/appendixes/famodiff.tex
changeset 871 0adb2c013880
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   234 The proof is similar to the proof of Corollary 1.3 of \cite{MR0283802}.
   234 The proof is similar to the proof of Corollary 1.3 of \cite{MR0283802}.
   235 
   235 
   236 Since $X$ is compact, we may assume without loss of generality that the cover $\cU$ is finite.
   236 Since $X$ is compact, we may assume without loss of generality that the cover $\cU$ is finite.
   237 Let $\cU = \{U_\alpha\}$, $1\le \alpha\le N$.
   237 Let $\cU = \{U_\alpha\}$, $1\le \alpha\le N$.
   238 
   238 
   239 We will need some wiggle room, so for each $\alpha$ choose a large finite number of open sets
   239 We will need some wiggle room, so for each $\alpha$ choose $2N$ additional open sets
   240 \[
   240 \[
   241 	U_\alpha = U_\alpha^0 \supset U_\alpha^1 \supset U_\alpha^2 \supset \cdots
   241 	U_\alpha = U_\alpha^0 \supset U_\alpha^\frac12 \supset U_\alpha^1 \supset U_\alpha^\frac32 \supset \cdots \supset U_\alpha^N
   242 \]
   242 \]
   243 so that for each fixed $i$ the sets $\{U_\alpha^i\}$ are an open cover of $X$, and also so that
   243 so that for each fixed $i$ the set $\cU^i = \{U_\alpha^i\}$ is an open cover of $X$, and also so that
   244 the closure $\ol{U_\alpha^i}$ is compact and $U_\alpha^{i-1} \supset \ol{U_\alpha^i}$.
   244 the closure $\ol{U_\alpha^i}$ is compact and $U_\alpha^{i-\frac12} \supset \ol{U_\alpha^i}$.
   245 \nn{say specifically how many we need?}
   245 %\nn{say specifically how many we need?}
   246 
       
   247 
   246 
   248 Let $P$ be some $k$-dimensional polyhedron and $f:P\to \Homeo(X)$.
   247 Let $P$ be some $k$-dimensional polyhedron and $f:P\to \Homeo(X)$.
   249 After subdividing $P$, we may assume that there exists $g\in \Homeo(X)$
   248 After subdividing $P$, we may assume that there exists $g\in \Homeo(X)$
   250 such that $g^{-1}\circ f(P)$ is contained in a small neighborhood of the 
   249 such that $g^{-1}\circ f(P)$ is contained in a small neighborhood of the 
   251 identity in $\Homeo(X)$.
   250 identity in $\Homeo(X)$.
   252 The sense of ``small" we mean will be explained below.
   251 The sense of ``small" we mean will be explained below.
   253 It depends only on $\cU$ and the choice of $U_\alpha^i$'s.
   252 It depends only on $\cU$ and the choice of $U_\alpha^i$'s.
   254 
   253 
   255 We may assume, inductively, that the restriction of $f$ to $\bd P$ is adapted to $\cU$.
   254 We may assume, inductively, that the restriction of $f$ to $\bd P$ is adapted to $\cU^N$.
   256 
   255 So $\bd P = \sum Q_\beta$, and the support of $f$ restricted to $Q_\beta$ is $V_\beta^N$, the union of $k-1$ of
       
   256 the $U_\alpha^N$'s.  Define $V_\beta^i \sup V_\beta^N$ to be the corresponding union of $k-1$
       
   257 of the $U_\alpha^i$'s.
       
   258 
       
   259 Define
       
   260 \[
       
   261 	W_j^i = U_1^i \cup U_2^i \cup \cdots \cup U_j^i .
       
   262 \]
       
   263 
       
   264 We will construct a sequence of maps $f_i : P\to \Homeo(X)$, for $i = 0, 1, \ldots, N$, with the following properties:
       
   265 \begin{itemize}
       
   266 \item[(A)] $f_0 = f$;
       
   267 \item[(B)] $f_i = g$ on $W_i^i$;
       
   268 \item[(C)] $f_i$ restricted to $Q_\beta$ has support contained in $V_\beta^{N-i}$; and
       
   269 \item[(D)] there is a homotopy $F_i : P\times I \to \Homeo(X)$ from $f_{i-1}$ to $f_i$ such that the 
       
   270 support of $F_i$ restricted to $Q_\beta\times I$ is contained in $U_i^i\cup V_\beta^{N-i}$.
       
   271 \nn{check this when done writing}
       
   272 \end{itemize}
       
   273 
       
   274 Once we have the $F_i$'s as in (D), we can finish the argument as follows.
       
   275 Assemble the $F_i$'s into a map $F: P\times [0,N] \to \Homeo(X)$.
       
   276 View $F$ as a homotopy rel boundary from $F$ restricted to $P\times\{0\}$ (which is just $f$ by (A))
       
   277 to $F$ restricted to $\bd P \times [0,N] \cup P\times\{N\}$.
       
   278 $F$ restricted to $\bd P \times [0,N]$ is adapted to $\cU$ by (D).
       
   279 $F$ restricted to $P\times\{N\}$ is constant on $W_N^N = X$ by (B), and therefore is, {\it a fortiori}, also adapted
       
   280 to $\cU$.
       
   281 
       
   282 \nn{resume revising here}
   257 
   283 
   258 Theorem 5.1 of \cite{MR0283802}, applied $N$ times, allows us
   284 Theorem 5.1 of \cite{MR0283802}, applied $N$ times, allows us
   259 to choose a homotopy $h:P\times [0,N] \to \Homeo(X)$ with the following properties:
   285 to choose a homotopy $h:P\times [0,N] \to \Homeo(X)$ with the following properties:
   260 \begin{itemize}
   286 \begin{itemize}
   261 \item $h(p, 0) = f(p)$ for all $p\in P$.
   287 \item $h(p, 0) = f(p)$ for all $p\in P$.