%!TEX root = ../blob1.tex

Let $CD_*(X, Y)$ denote $C_*(\Diff(X \to Y))$, the singular chain complex of

the space of diffeomorphisms

\nn{or homeomorphisms}

between the $n$-manifolds $X$ and $Y$ (extending a fixed diffeomorphism $\bd X \to \bd Y$).

For convenience, we will permit the singular cells generating $CD_*(X, Y)$ to be more general

than simplices --- they can be based on any linear polyhedron.

\nn{be more restrictive here?  does more need to be said?}

We also will use the abbreviated notation $CD_*(X) \deq CD_*(X, X)$.

\begin{prop}  \label{CDprop}

For $n$-manifolds $X$ and $Y$ there is a chain map

\eq{

    e_{XY} : CD_*(X, Y) \otimes \bc_*(X) \to \bc_*(Y) .

}

On $CD_0(X, Y) \otimes \bc_*(X)$ it agrees with the obvious action of $\Diff(X, Y)$ on $\bc_*(X)$

(Proposition (\ref{diff0prop})).

For any splittings $X = X_1 \cup X_2$ and $Y = Y_1 \cup Y_2$, 

the following diagram commutes up to homotopy

\eq{ \xymatrix{

     CD_*(X, Y) \otimes \bc_*(X) \ar[r]^{e_{XY}}    & \bc_*(Y) \\

     CD_*(X_1, Y_1) \otimes CD_*(X_2, Y_2) \otimes \bc_*(X_1) \otimes \bc_*(X_2)

        \ar@/_4ex/[r]_{e_{X_1Y_1} \otimes e_{X_2Y_2}}  \ar[u]^{\gl \otimes \gl}  &

            \bc_*(Y_1) \otimes \bc_*(Y_2) \ar[u]_{\gl}

} }

Any other map satisfying the above two properties is homotopic to $e_X$.

\end{prop}

\nn{need to rewrite for self-gluing instead of gluing two pieces together}

\nn{Should say something stronger about uniqueness.

Something like: there is

a contractible subcomplex of the complex of chain maps

$CD_*(X) \otimes \bc_*(X) \to \bc_*(X)$ (0-cells are the maps, 1-cells are homotopies, etc.),

and all choices in the construction lie in the 0-cells of this

contractible subcomplex.

Or maybe better to say any two choices are homotopic, and

any two homotopies and second order homotopic, and so on.}

\nn{Also need to say something about associativity.

Put it in the above prop or make it a separate prop?

I lean toward the latter.}

\medskip

The proof will occupy the remainder of this section.

\nn{unless we put associativity prop at end}

Without loss of generality, we will assume $X = Y$.

\medskip

Let $f: P \times X \to X$ be a family of diffeomorphisms and $S \sub X$.

We say that {\it $f$ is supported on $S$} if $f(p, x) = f(q, x)$ for all

$x \notin S$ and $p, q \in P$. Equivalently, $f$ is supported on $S$ if there is a family of diffeomorphisms $f' : P \times S \to S$ and a `background'

diffeomorphism $f_0 : X \to X$ so that

\begin{align}

	f(p,s) & = f_0(f'(p,s)) \;\;\;\; \mbox{for}\; (p, s) \in P\times S \\

\intertext{and}

	f(p,x) & = f_0(x) \;\;\;\; \mbox{for}\; (p, x) \in {P \times (X \setmin S)}.

\end{align}

Note that if $f$ is supported on $S$ then it is also supported on any $R \sup S$.

Let $\cU = \{U_\alpha\}$ be an open cover of $X$.

A $k$-parameter family of diffeomorphisms $f: P \times X \to X$ is

{\it adapted to $\cU$} if there is a factorization

\eq{

    P = P_1 \times \cdots \times P_m

}

(for some $m \le k$)

and families of diffeomorphisms

\eq{

    f_i :  P_i \times X \to X

}

such that

\begin{itemize}

\item each $f_i$ is supported on some connected $V_i \sub X$;

\item the sets $V_i$ are mutually disjoint;

\item each $V_i$ is the union of at most $k_i$ of the $U_\alpha$'s,

where $k_i = \dim(P_i)$; and

\item $f(p, \cdot) = g \circ f_1(p_1, \cdot) \circ \cdots \circ f_m(p_m, \cdot)$

for all $p = (p_1, \ldots, p_m)$, for some fixed $g \in \Diff(X)$.

\end{itemize}

A chain $x \in CD_k(X)$ is (by definition) adapted to $\cU$ if it is the sum

of singular cells, each of which is adapted to $\cU$.

(Actually, in this section we will only need families of diffeomorphisms to be 

{\it weakly adapted} to $\cU$, meaning that the support of $f$ is contained in the union

of at most $k$ of the $U_\alpha$'s.)

\begin{lemma}  \label{extension_lemma}

Let $x \in CD_k(X)$ be a singular chain such that $\bd x$ is adapted to $\cU$.

Then $x$ is homotopic (rel boundary) to some $x' \in CD_k(X)$ which is adapted to $\cU$.

Furthermore, one can choose the homotopy so that its support is equal to the support of $x$.

\end{lemma}

The proof will be given in Section \ref{sec:localising}.

\medskip

Before diving into the details, we outline our strategy for the proof of Proposition \ref{CDprop}.

%Suppose for the moment that evaluation maps with the advertised properties exist.

Let $p$ be a singular cell in $CD_k(X)$ and $b$ be a blob diagram in $\bc_*(X)$.

Suppose that there exists $V \sub X$ such that

\begin{enumerate}

\item $V$ is homeomorphic to a disjoint union of balls, and

\item $\supp(p) \cup \supp(b) \sub V$.

\end{enumerate}

Let $W = X \setmin V$, and let $V' = p(V)$ and $W' = p(W)$.

We then have a factorization 

\[

	p = \gl(q, r),

\]

where $q \in CD_k(V, V')$ and $r' \in CD_0(W, W')$.

We can also factorize $b = \gl(b_V, b_W)$, where $b_V\in \bc_*(V)$ and $b_W\in\bc_0(W)$.

According to the commutative diagram of the proposition, we must have

\[

	e_X(p\otimes b) = e_X(\gl(q\otimes b_V, r\otimes b_W)) = 

				gl(e_{VV'}(q\otimes b_V), e_{WW'}(r\otimes b_W)) .

\]

Since $r$ is a plain, 0-parameter family of diffeomorphisms, we must have

\[

	e_{WW'}(r\otimes b_W) = r(b_W),

\]

where $r(b_W)$ denotes the obvious action of diffeomorphisms on blob diagrams (in

this case a 0-blob diagram).

Since $V'$ is a disjoint union of balls, $\bc_*(V')$ is acyclic in degrees $>0$ 

(by \ref{disjunion} and \ref{bcontract}).

Assuming inductively that we have already defined $e_{VV'}(\bd(q\otimes b_V))$,

there is, up to (iterated) homotopy, a unique choice for $e_{VV'}(q\otimes b_V)$

such that 

\[

	\bd(e_{VV'}(q\otimes b_V)) = e_{VV'}(\bd(q\otimes b_V)) .

\]

Thus the conditions of the proposition determine (up to homotopy) the evaluation

map for generators $p\otimes b$ such that $\supp(p) \cup \supp(b)$ is contained in a disjoint

union of balls.

On the other hand, Lemma \ref{extension_lemma} allows us to homotope 

\nn{is this commonly used as a verb?} arbitrary generators to sums of generators with this property.

\nn{should give a name to this property; also forward reference}

This (roughly) establishes the uniqueness part of the proposition.

To show existence, we must show that the various choices involved in constructing

evaluation maps in this way affect the final answer only by a homotopy.

\nn{maybe put a little more into the outline before diving into the details.}

\nn{Note: At the moment this section is very inconsistent with respect to PL versus smooth,

homeomorphism versus diffeomorphism, etc.

We expect that everything is true in the PL category, but at the moment our proof

avails itself to smooth techniques.

Furthermore, it traditional in the literature to speak of $C_*(\Diff(X))$

rather than $C_*(\Homeo(X))$.}

\medskip

Now for the details.

Notation: Let $|b| = \supp(b)$, $|p| = \supp(p)$.

Choose a metric on $X$.

Choose a monotone decreasing sequence of positive real numbers $\ep_i$ converging to zero

(e.g.\ $\ep_i = 2^{-i}$).

Choose another sequence of positive real numbers $\delta_i$ such that $\delta_i/\ep_i$

converges monotonically to zero (e.g.\ $\delta_i = \ep_i^2$).

Given a generator $p\otimes b$ of $CD_*(X)\otimes \bc_*(X)$ and non-negative integers $i$ and $k$

define

\[

	N_{i,k}(p\ot b) \deq \Nbd_{k\ep_i}(|b|) \cup \Nbd_{4^k\delta_i}(|p|).

\]

In other words, we use the metric to choose nested neighborhoods of $|b|\cup |p|$ (parameterized

by $k$), with $\ep_i$ controlling the size of the buffer around $|b|$ and $\delta_i$ controlling

the size of the buffer around $|p|$.

(The $4^k$ comes from Lemma \ref{xxxx}.)

Next we define subcomplexes $G_*^{i,m} \sub CD_*(X)\otimes \bc_*(X)$.

Let $p\ot b$ be a generator of $CD_*(X)\otimes \bc_*(X)$ and let $k = \deg(p\ot b)

= \deg(p) + \deg(b)$.

$p\ot b$ is (by definition) in $G_*^{i,m}$ if either (a) $\deg(p) = 0$ or (b)

there exist codimension-zero submanifolds $V_0,\ldots,V_m \sub X$ such that each $V_j$

is homeomorphic to a disjoint union of balls and

\[

	N_{i,k}(p\ot b) \subeq V_0 \subeq N_{i,k+1}(p\ot b)

			\subeq V_1 \subeq \cdots \subeq V_m \subeq N_{i,k+m+1}(p\ot b) .

\]

Further, we require (inductively) that $\bd(p\ot b) \in G_*^{i,m}$.

We also require that $b$ is splitable (transverse) along the boundary of each $V_l$.

Note that $G_*^{i,m+1} \subeq G_*^{i,m}$.

As sketched above and explained in detail below, 

$G_*^{i,m}$ is a subcomplex where it is easy to define

the evaluation map.

The parameter $m$ controls the number of iterated homotopies we are able to construct

The larger $i$ is (i.e.\ the smaller $\ep_i$ is), the better $G_*^{i,m}$ approximates all of

Next we define a chain map (dependent on some choices) $e: G_*^{i,m} \to \bc_*(X)$.

Let $p\ot b \in G_*^{i,m}$.

If $\deg(p) = 0$, define

\[

	e(p\ot b) = p(b) ,

\]

where $p(b)$ denotes the obvious action of the diffeomorphism(s) $p$ on the blob diagram $b$.

For general $p\ot b$ ($\deg(p) \ge 1$) assume inductively that we have already defined

$e(p'\ot b')$ when $\deg(p') + \deg(b') < k = \deg(p) + \deg(b)$.

Choose $V = V_0$ as above so that 

\[

	N_{i,k}(p\ot b) \subeq V \subeq N_{i,k+1}(p\ot b) .

\]

Let $\bd(p\ot b) = \sum_j p_j\ot b_j$, and let $V^j$ be the choice of neighborhood

of $|p_j|\cup |b_j|$ made at the preceding stage of the induction.

For all $j$, 

\[

	V^j \subeq N_{i,(k-1)+1}(p_j\ot b_j) \subeq N_{i,k}(p\ot b) \subeq V .

\]

(The second inclusion uses the facts that $|p_j| \subeq |p|$ and $|b_j| \subeq |b|$.)

We therefore have splittings

\[

	p = p'\bullet p'' , \;\; b = b'\bullet b'' , \;\; e(\bd(p\ot b)) = f'\bullet b'' ,

\]

where $p' \in CD_*(V)$, $p'' \in CD_*(X\setmin V)$, 

$b' \in \bc_*(V)$, $b'' \in \bc_*(X\setmin V)$, 

$f' \in \bc_*(p(V))$, and $f'' \in \bc_*(p(X\setmin V))$.

(Note that since the family of diffeomorphisms $p$ is constant (independent of parameters)

near $\bd V$, the expressions $p(V) \sub X$ and $p(X\setmin V) \sub X$ are

unambiguous.)

We have $\deg(p'') = 0$ and, inductively, $f'' = p''(b'')$.

%We also have that $\deg(b'') = 0 = \deg(p'')$.

Choose $x' \in \bc_*(p(V))$ such that $\bd x' = f'$.

This is possible by \nn{...}.

Finally, define

\[

	e(p\ot b) \deq x' \bullet p''(b'') .

\]

Note that above we are essentially using the method of acyclic models.

For each generator $p\ot b$ we specify the acyclic (in positive degrees) 

target complex $\bc_*(p(V)) \bullet p''(b'')$.

The definition of $e: G_*^{i,m} \to \bc_*(X)$ depends on two sets of choices:

The choice of neighborhoods $V$ and the choice of inverse boundaries $x'$.

The next two lemmas show that up to (iterated) homotopy $e$ is independent

of these choices.

\begin{lemma}

Let $\tilde{e} :  G_*^{i,m} \to \bc_*(X)$ be a chain map constructed like $e$ above, but with

different choices of $x'$ at each step.

(Same choice of $V$ at each step.)

Then $e$ and $\tilde{e}$ are homotopic via a homotopy in $\bc_*(p(V)) \bullet p''(b'')$.

Any two choices of such a first-order homotopy are second-order homotopic, and so on, 

to arbitrary order.

\end{lemma}

\begin{proof}

This is a standard result in the method of acyclic models.

\nn{should we say more here?}

\nn{maybe this lemma should be subsumed into the next lemma.  probably it should.}

\end{proof}

Let $\tilde{e} :  G_*^{i,m} \to \bc_*(X)$ be a chain map constructed like $e$ above, but with

different choices of $V$ (and hence also different choices of $x'$) at each step.

If $m \ge 1$ then $e$ and $\tilde{e}$ are homotopic.

If $m \ge 2$ then any two choices of this first-order homotopy are second-order homotopic.

And so on.

In other words,  $e :  G_*^{i,m} \to \bc_*(X)$ is well-defined up to $m$-th order homotopy.

\end{lemma}

\begin{proof}

We construct $h: G_*^{i,m} \to \bc_*(X)$ such that $\bd h + h\bd = e - \tilde{e}$.

$e$ and $\tilde{e}$ coincide on bidegrees $(0, j)$, so define $h$

to be zero there.

Assume inductively that $h$ has been defined for degrees less than $k$.

Let $p\ot b$ be a generator of degree $k$.

Choose $V_1$ as in the definition of $G_*^{i,m}$ so that

\[

	N_{i,k+1}(p\ot b) \subeq V_1 \subeq N_{i,k+2}(p\ot b) .

\]

There are splittings

\[

	p = p'_1\bullet p''_1 , \;\; b = b'_1\bullet b''_1 , 

			\;\; e(p\ot b) - \tilde{e}(p\ot b) - h(\bd(p\ot b)) = f'_1\bullet f''_1 ,

\]

where $p'_1 \in CD_*(V_1)$, $p''_1 \in CD_*(X\setmin V_1)$, 

$b'_1 \in \bc_*(V_1)$, $b''_1 \in \bc_*(X\setmin V_1)$, 

$f'_1 \in \bc_*(p(V_1))$, and $f''_1 \in \bc_*(p(X\setmin V_1))$.

Inductively, $\bd f'_1 = 0$.

Choose $x'_1 \in \bc_*(p(V_1))$ so that $\bd x'_1 = f'_1$.

Define 

\[

	h(p\ot b) \deq x'_1 \bullet p''_1(b''_1) .

\]

This completes the construction of the first-order homotopy when $m \ge 1$.

The $j$-th order homotopy is constructed similarly, with $V_j$ replacing $V_1$ above.

\end{proof}

Note that on $G_*^{i,m+1} \subeq G_*^{i,m}$, we have defined two maps,

call them $e_{i,m}$ and $e_{i,m+1}$.

An easy variation on the above lemma shows that $e_{i,m}$ and $e_{i,m+1}$ are $m$-th 

order homotopic.

Next we show how to homotope chains in $CD_*(X)\ot \bc_*(X)$ to one of the 

$G_*^{i,m}$.

Choose a monotone decreasing sequence of real numbers $\gamma_j$ converging to zero.

Let $\cU_j$ denote the open cover of $X$ by balls of radius $\gamma_j$.

Let $h_j: CD_*(X)\to CD_*(X)$ be a chain map homotopic to the identity whose image is spanned by diffeomorphisms with support compatible with $\cU_j$, as described in Lemma \ref{xxxxx}.

Recall that $h_j$ and also the homotopy connecting it to the identity do not increase

supports.

Define

\[

	g_j \deq h_j\circ h_{j-1} \circ \cdots \circ h_1 .

\]

The next lemma says that for all generators $p\ot b$ we can choose $j$ large enough so that

$g_j(p)\ot b$ lies in $G_*^{i,m}$, for arbitrary $m$ and sufficiently large $i$ 

(depending on $b$, $n = \deg(p)$ and $m$).

\nn{not the same $n$ as the dimension of the manifolds; fix this}

Fix a blob diagram $b$, a homotopy order $m$ and a degree $n$ for $CD_*(X)$.

Then there exists a constant $k_{bmn}$ such that for all $i \ge k_{bmn}$

there exists another constant $j_i$ such that for all $j \ge j_i$ and all $p\in CD_n(X)$ 

we have $g_j(p)\ot b \in G_*^{i,m}$.

\end{lemma}

\begin{proof}

Let $c$ be a subset of the blobs of $b$.

There exists $l > 0$ such that $\Nbd_u(c)$ is homeomorphic to $|c|$ for all $u < l$ 

and all such $c$.

(Here we are using a piecewise smoothness assumption for $\bd c$, and also

the fact that $\bd c$ is collared.)

Let $r = \deg(b)$ and 

\[

	t = r+n+m+1 .

\]

Choose $k = k_{bmn}$ such that

\[

	t\ep_k < l

\]

and

\[

	n\cdot ( 4^t \delta_i) < \ep_k/3 .

\]

Let $i \ge k_{bmn}$.

Choose $j = j_i$ so that

\[

	t\gamma_j < \ep_i/3

\]

and also so that $\gamma_j$ is less than the constant $\eta(X, m, k)$ of Lemma \ref{xxyy5}.

\nn{...}

\end{proof}

In the next few lemmas we have made no effort to optimize the various bounds.

(The bounds are, however, optimal in the sense of minimizing the amount of work

we do.  Equivalently, they are the first bounds we thought of.)

We say that a subset $S$ of a metric space has radius $\le r$ if $S$ is contained in

some metric ball of radius $r$.

\begin{lemma}

Let $S \sub \ebb^n$ (Euclidean $n$-space) have radius $\le r$.  

Then $\Nbd_a(S)$ is homeomorphic to a ball for $a \ge 2r$.

\end{lemma}

\begin{proof} \label{xxyy2}

Let $S$ be contained in $B_r(y)$, $y \in \ebb^n$.

Note that $\Nbd_a(S) \sup B_r(y)$.

Simple applications of the triangle inequality show that $\Nbd_a(S)$ 

is star-shaped with respect to $y$.

\end{proof}

\begin{lemma} \label{xxyy3}

Let $S \sub \ebb^n$ be contained in a union (not necessarily disjoint)

of $k$ metric balls of radius $r$.

Then there exists a neighborhood $U$ of $S$ such that $U$ is homeomorphic to a disjoint union

of balls and

\[

	\Nbd_{2r}(S) \subeq U \subeq \Nbd_{4^k r}(S) .

\]

\end{lemma}

\begin{proof}

Partition $S$ into $k$ disjoint subsets $S_1,\ldots,S_k$, each of radius $\le r$.

By Lemma \ref{xxyy2}, each $\Nbd(S_i)$ is homeomorphic to a ball.

If these balls are disjoint (always the case if $k=1$) we are done.

If two (or more) of them intersect, then $S$ is contained in a union of $k-1$ metric

balls of radius $4r$.

By induction, there is a neighborhood $U$ of $S$ such that

\[

	U \subeq \Nbd_{4^{k-1}\cdot4r} .

\]

\end{proof}

\begin{lemma} \label{xxyy4}

Let $S \sub \ebb^n$ be contained in a union (not necessarily disjoint)

of $k$ metric balls of radius $r$.

Then there exist neighborhoods $U_0, U_1, U_2, \ldots$ of $S$,

each homeomorphic to a disjoint union of balls, such that

\[

	\Nbd_{2r}(S) \subeq U_0 \subeq \Nbd_{4^k r}(S)

		\subeq U_1 \subeq \Nbd_{4^{2k} r}(S)

		\subeq U_2 \subeq \Nbd_{4^{3k} r}(S) \cdots

\]

\end{lemma}

\begin{proof}

Apply Lemma \ref {xxyy3} repeatedly.

\end{proof}

\begin{lemma} \label{xxyy5}

Let $M$ be a Riemannian $n$-manifold and positive integers $m$ and $k$.

There exists a constant $\eta(M, m, k)$ such that for all subsets

$S\subeq M$ which are contained in a (not necessarily disjoint) union of

$k$ metric balls of radius $r$, $r < \eta(M, m, k)$, 

there exist neighborhoods $U_0, U_1, \ldots, U_m$ of $S$,

each homeomorphic to a disjoint union of balls, such that

\[

	\Nbd_{2r}(S) \subeq U_0 \subeq \Nbd_{4^k r}(S)

		\subeq U_1 \subeq \Nbd_{4^{2k} r}(S) \cdots

		\subeq U_m \subeq \Nbd_{4^{(m+1)k} r}(S) .

\]

\end{lemma}

\begin{proof}

Choose $\eta = \eta(M, m, k)$ small enough so that metric balls of radius $4^{(m+1)k} \eta$

are injective and also have small distortion with respect to a Euclidean metric.

Then proceed as in the proof of Lemma \ref{xxyy4}.

\end{proof}

\medskip

\noop{

\begin{lemma}

\end{lemma}

\begin{proof}

\end{proof}

}

\nn{to be continued....}

%\nn{say something about associativity here}