%!TEX root = ../blob1.tex

\section{Families of Diffeomorphisms}  \label{sec:localising}

Lo, the proof of Lemma (\ref{extension_lemma}):

\nn{should this be an appendix instead?}

\nn{for pedagogical reasons, should do $k=1,2$ cases first; probably do this in

later draft}

\nn{not sure what the best way to deal with boundary is; for now just give main argument, worry

about boundary later}

Recall that we are given

an open cover $\cU = \{U_\alpha\}$ and an

$x \in CD_k(X)$ such that $\bd x$ is adapted to $\cU$.

We must find a homotopy of $x$ (rel boundary) to some $x' \in CD_k(X)$ which is adapted to $\cU$.

Let $\{r_\alpha : X \to [0,1]\}$ be a partition of unity for $\cU$.

As a first approximation to the argument we will eventually make, let's replace $x$

with a single singular cell

\eq{

    f: P \times X \to X .

}

Also, we'll ignore for now issues around $\bd P$.

Our homotopy will have the form

\eqar{

    F: I \times P \times X &\to& X \\

    (t, p, x) &\mapsto& f(u(t, p, x), x)

}

for some function

\eq{

    u : I \times P \times X \to P .

}

First we describe $u$, then we argue that it does what we want it to do.

For each cover index $\alpha$ choose a cell decomposition $K_\alpha$ of $P$.

The various $K_\alpha$ should be in general position with respect to each other.

We will see below that the $K_\alpha$'s need to be sufficiently fine in order

to insure that $F$ above is a homotopy through diffeomorphisms of $X$ and not

merely a homotopy through maps $X\to X$.

Let $L$ be the union of all the $K_\alpha$'s.

$L$ is itself a cell decomposition of $P$.

\nn{next two sentences not needed?}

To each cell $a$ of $L$ we associate the tuple $(c_\alpha)$,

where $c_\alpha$ is the codimension of the cell of $K_\alpha$ which contains $c$.

Since the $K_\alpha$'s are in general position, we have $\sum c_\alpha \le k$.

Let $J$ denote the handle decomposition of $P$ corresponding to $L$.

Each $i$-handle $C$ of $J$ has an $i$-dimensional tangential coordinate and,

more importantly, a $k{-}i$-dimensional normal coordinate.

For each (top-dimensional) $k$-cell $c$ of each $K_\alpha$, choose a point $p_c \in c \sub P$.

Let $D$ be a $k$-handle of $J$, and let $D$ also denote the corresponding

$k$-cell of $L$.

To $D$ we associate the tuple $(c_\alpha)$ of $k$-cells of the $K_\alpha$'s

which contain $d$, and also the corresponding tuple $(p_{c_\alpha})$ of points in $P$.

For $p \in D$ we define

\eq{

    u(t, p, x) = (1-t)p + t \sum_\alpha r_\alpha(x) p_{c_\alpha} .

}

(Recall that $P$ is a single linear cell, so the weighted average of points of $P$

makes sense.)

So far we have defined $u(t, p, x)$ when $p$ lies in a $k$-handle of $J$.

For handles of $J$ of index less than $k$, we will define $u$ to

interpolate between the values on $k$-handles defined above.

If $p$ lies in a $k{-}1$-handle $E$, let $\eta : E \to [0,1]$ be the normal coordinate

of $E$.

In particular, $\eta$ is equal to 0 or 1 only at the intersection of $E$

with a $k$-handle.

Let $\beta$ be the index of the $K_\beta$ containing the $k{-}1$-cell

corresponding to $E$.

Let $q_0, q_1 \in P$ be the points associated to the two $k$-cells of $K_\beta$

adjacent to the $k{-}1$-cell corresponding to $E$.

For $p \in E$, define

\eq{

    u(t, p, x) = (1-t)p + t \left( \sum_{\alpha \ne \beta} r_\alpha(x) p_{c_\alpha}

            + r_\beta(x) (\eta(p) q_1 + (1-\eta(p)) q_0) \right) .

}

In general, for $E$ a $k{-}j$-handle, there is a normal coordinate

$\eta: E \to R$, where $R$ is some $j$-dimensional polyhedron.

The vertices of $R$ are associated to $k$-cells of the $K_\alpha$, and thence to points of $P$.

If we triangulate $R$ (without introducing new vertices), we can linearly extend

a map from the vertices of $R$ into $P$ to a map of all of $R$ into $P$.

Let $\cN$ be the set of all $\beta$ for which $K_\beta$ has a $k$-cell whose boundary meets

the $k{-}j$-cell corresponding to $E$.

For each $\beta \in \cN$, let $\{q_{\beta i}\}$ be the set of points in $P$ associated to the aforementioned $k$-cells.

Now define, for $p \in E$,

\eq{

    u(t, p, x) = (1-t)p + t \left(

            \sum_{\alpha \notin \cN} r_\alpha(x) p_{c_\alpha}

                + \sum_{\beta \in \cN} r_\beta(x) \left( \sum_i \eta_{\beta i}(p) \cdot q_{\beta i} \right)

             \right) .

}

Here $\eta_{\beta i}(p)$ is the weight given to $q_{\beta i}$ by the linear extension

mentioned above.

This completes the definition of $u: I \times P \times X \to P$.

\medskip

Next we verify that $u$ has the desired properties.

Since $u(0, p, x) = p$ for all $p\in P$ and $x\in X$, $F(0, p, x) = f(p, x)$ for all $p$ and $x$.

Therefore $F$ is a homotopy from $f$ to something.

Next we show that if the $K_\alpha$'s are sufficiently fine cell decompositions,

then $F$ is a homotopy through diffeomorphisms.

We must show that the derivative $\pd{F}{x}(t, p, x)$ is non-singular for all $(t, p, x)$.

We have

\eq{

%   \pd{F}{x}(t, p, x) = \pd{f}{x}(u(t, p, x), x) + \pd{f}{p}(u(t, p, x), x) \pd{u}{x}(t, p, x) .

    \pd{F}{x} = \pd{f}{x} + \pd{f}{p} \pd{u}{x} .

}

Since $f$ is a family of diffeomorphisms, $\pd{f}{x}$ is non-singular and

\nn{bounded away from zero, or something like that}.

(Recall that $X$ and $P$ are compact.)

Also, $\pd{f}{p}$ is bounded.

So if we can insure that $\pd{u}{x}$ is sufficiently small, we are done.

It follows from Equation xxxx above that $\pd{u}{x}$ depends on $\pd{r_\alpha}{x}$

(which is bounded)

and the differences amongst the various $p_{c_\alpha}$'s and $q_{\beta i}$'s.

These differences are small if the cell decompositions $K_\alpha$ are sufficiently fine.

This completes the proof that $F$ is a homotopy through diffeomorphisms.

\medskip

Next we show that for each handle $D \sub P$, $F(1, \cdot, \cdot) : D\times X \to X$

is a singular cell adapted to $\cU$.

This will complete the proof of the lemma.

\nn{except for boundary issues and the `$P$ is a cell' assumption}

Let $j$ be the codimension of $D$.

(Or rather, the codimension of its corresponding cell.  From now on we will not make a distinction

between handle and corresponding cell.)

Then $j = j_1 + \cdots + j_m$, $0 \le m \le k$,

where the $j_i$'s are the codimensions of the $K_\alpha$

cells of codimension greater than 0 which intersect to form $D$.

We will show that

if the relevant $U_\alpha$'s are disjoint, then

$F(1, \cdot, \cdot) : D\times X \to X$

is a product of singular cells of dimensions $j_1, \ldots, j_m$.

If some of the relevant $U_\alpha$'s intersect, then we will get a product of singular

cells whose dimensions correspond to a partition of the $j_i$'s.

We will consider some simple special cases first, then do the general case.

First consider the case $j=0$ (and $m=0$).

A quick look at Equation xxxx above shows that $u(1, p, x)$, and hence $F(1, p, x)$,

is independent of $p \in P$.

So the corresponding map $D \to \Diff(X)$ is constant.

Next consider the case $j = 1$ (and $m=1$, $j_1=1$).

Now Equation yyyy applies.

We can write $D = D'\times I$, where the normal coordinate $\eta$ is constant on $D'$.

It follows that the singular cell $D \to \Diff(X)$ can be written as a product

of a constant map $D' \to \Diff(X)$ and a singular 1-cell $I \to \Diff(X)$.

The singular 1-cell is supported on $U_\beta$, since $r_\beta = 0$ outside of this set.

Next case: $j=2$, $m=1$, $j_1 = 2$.

This is similar to the previous case, except that the normal bundle is 2-dimensional instead of

1-dimensional.

We have that $D \to \Diff(X)$ is a product of a constant singular $k{-}2$-cell

and a 2-cell with support $U_\beta$.

Next case: $j=2$, $m=2$, $j_1 = j_2 = 1$.

In this case the codimension 2 cell $D$ is the intersection of two

codimension 1 cells, from $K_\beta$ and $K_\gamma$.

We can write $D = D' \times I \times I$, where the normal coordinates are constant

on $D'$, and the two $I$ factors correspond to $\beta$ and $\gamma$.

If $U_\beta$ and $U_\gamma$ are disjoint, then we can factor $D$ into a constant $k{-}2$-cell and

two 1-cells, supported on $U_\beta$ and $U_\gamma$ respectively.

If $U_\beta$ and $U_\gamma$ intersect, then we can factor $D$ into a constant $k{-}2$-cell and

a 2-cell supported on $U_\beta \cup U_\gamma$.

\nn{need to check that this is true}

\nn{finally, general case...}

\nn{this completes proof}

\input{text/explicit.tex}