%!TEX root = ../blob1.tex

\section{The blob complex for $A_\infty$ $n$-categories}

\label{sec:ainfblob}

Given an $A_\infty$ $n$-category $\cC$ and an $n$-manifold $M$, we define the blob

complex $\bc_*(M)$ to the be the homotopy colimit $\cC(M)$ of Section \ref{sec:ncats}.

\nn{say something about this being anticlimatically tautological?}

We will show below 

in Corollary \ref{cor:new-old}

that this agrees (up to homotopy) with our original definition of the blob complex

in the case of plain $n$-categories.

When we need to distinguish between the new and old definitions, we will refer to the 

new-fangled and old-fashioned blob complex.

\medskip

An important technical tool in the proofs of this section is provided by the idea of `small blobs'.

Fix $\cU$, an open cover of $M$. Define the `small blob complex' $\bc^{\cU}_*(M)$ to be the subcomplex of $\bc_*(M)$ of all blob diagrams in which every blob is contained in some open set of $\cU$. 

\nn{KW: We need something a little stronger: Every blob diagram (even a 0-blob diagram) is splittable into pieces which are small w.r.t.\ $\cU$.

If field have potentially large coupons/boxes, then this is a non-trivial constraint.

On the other hand, we could probably get away with ignoring this point.

Maybe the exposition will be better if we sweep this technical detail under the rug?}

\begin{thm}[Small blobs] \label{thm:small-blobs}

The inclusion $i: \bc^{\cU}_*(M) \into \bc_*(M)$ is a homotopy equivalence.

\end{thm}

The proof appears in \S \ref{appendix:small-blobs}.

\subsection{A product formula}

\label{ss:product-formula}

\noop{

Let $Y$ be a $k$-manifold, $F$ be an $n{-}k$-manifold, and 

\[

	E = Y\times F .

\]

Let $\cC$ be an $n$-category.

Let $\cF$ be the $k$-category of Example \ref{ex:blob-complexes-of-balls}, 

\[

	\cF(X) = \cC(X\times F)

\]

for $X$ an $m$-ball with $m\le k$.

}

\nn{need to settle on notation; proof and statement are inconsistent}

\begin{thm} \label{product_thm}

Given a topological $n$-category $C$ and a $n{-}k$-manifold $F$, recall from Example \ref{ex:blob-complexes-of-balls} that there is an  $A_\infty$ $k$-category $C^{\times F}$ defined by

\begin{equation*}

C^{\times F}(B) = \cB_*(B \times F, C).

\end{equation*}

Now, given a $k$-manifold $Y$, there is a homotopy equivalence between the `old-fashioned' blob complex for $Y \times F$ with coefficients in $C$ and the `new-fangled' (i.e.\ homotopy colimit) blob complex for $Y$ with coefficients in $C^{\times F}$:

\begin{align*}

\cB_*(Y \times F, C) & \htpy \cB_*(Y, C^{\times F})

\end{align*}

\end{thm}

\begin{proof}%[Proof of Theorem \ref{product_thm}]

We will use the concrete description of the colimit from Subsection \ref{ss:ncat_fields}.

First we define a map 

\[

	\psi: \bc_*^\cF(Y) \to \bc_*^C(Y\times F) .

\]

In filtration degree 0 we just glue together the various blob diagrams on $X_i\times F$

(where $X_i$ is a component of a permissible decomposition of $Y$) to get a blob diagram on

$Y\times F$.

In filtration degrees 1 and higher we define the map to be zero.

It is easy to check that this is a chain map.

In the other direction, we will define a subcomplex $G_*\sub \bc_*^C(Y\times F)$

and a map

\[

	\phi: G_* \to \bc_*^\cF(Y) .

\]

Given a decomposition $K$ of $Y$ into $k$-balls $X_i$, let $K\times F$ denote the corresponding

decomposition of $Y\times F$ into the pieces $X_i\times F$.

Let $G_*\sub \bc_*^C(Y\times F)$ be the subcomplex generated by blob diagrams $a$ such that there

exists a decomposition $K$ of $Y$ such that $a$ splits along $K\times F$.

It follows from Proposition \ref{thm:small-blobs} that $\bc_*^C(Y\times F)$ is homotopic to a subcomplex of $G_*$.

(If the blobs of $a$ are small with respect to a sufficiently fine cover then their

projections to $Y$ are contained in some disjoint union of balls.)

Note that the image of $\psi$ is equal to $G_*$.

We will define $\phi: G_* \to \bc_*^\cF(Y)$ using the method of acyclic models.

Let $a$ be a generator of $G_*$.

Let $D(a)$ denote the subcomplex of $\bc_*^\cF(Y)$ generated by all $(b, \ol{K})$

such that $a$ splits along $K_0\times F$ and $b$ is a generator appearing

in an iterated boundary of $a$ (this includes $a$ itself).

(Recall that $\ol{K} = (K_0,\ldots,K_l)$ denotes a chain of decompositions;

see Subsection \ref{ss:ncat_fields}.)

By $(b, \ol{K})$ we really mean $(b^\sharp, \ol{K})$, where $b^\sharp$ is 

$b$ split according to $K_0\times F$.

To simplify notation we will just write plain $b$ instead of $b^\sharp$.

Roughly speaking, $D(a)$ consists of filtration degree 0 stuff which glues up to give

$a$ (or one of its iterated boundaries), filtration degree 1 stuff which makes all of the filtration degree 0 stuff homologous, 

filtration degree 2 stuff which kills the homology created by the 

filtration degree 1 stuff, and so on.

More formally,

\begin{lemma}

$D(a)$ is acyclic.

\end{lemma}

\begin{proof}

We will prove acyclicity in the first couple of degrees, and \nn{in this draft, at least}

leave the general case to the reader.

Let $K$ and $K'$ be two decompositions of $Y$ compatible with $a$.

We want to show that $(a, K)$ and $(a, K')$ are homologous via filtration degree 1 stuff.

\nn{need to say this better; these two chains don't have the same boundary.}

We might hope that $K$ and $K'$ have a common refinement, but this is not necessarily

the case.

(Consider the $x$-axis and the graph of $y = x^2\sin(1/x)$ in $\r^2$.)

However, we {\it can} find another decomposition $L$ such that $L$ shares common

refinements with both $K$ and $K'$.

Let $KL$ and $K'L$ denote these two refinements.

Then filtration degree 1 chains associated to the four anti-refinemnts

$KL\to K$, $KL\to L$, $K'L\to L$ and $K'L\to K'$

give the desired chain connecting $(a, K)$ and $(a, K')$

(see Figure \ref{zzz4}).

\begin{figure}[!ht]

\begin{equation*}

\begin{tikzpicture}

\foreach \x/\label in {-3/K, 0/L, 3/K'} {

	\node(\label) at (\x,0) {$\label$};

}

\foreach \x/\la/\lb in {-1.5/K/L, 1.5/K'/L} {

	\node(\la \lb) at (\x,-1.5) {$\la \lb$};

	\draw[->] (\la \lb) -- (\la);

	\draw[->] (\la \lb) -- (\lb); 

}

\end{tikzpicture}

\end{equation*}

\caption{Connecting $K$ and $K'$ via $L$}

\label{zzz4}

\end{figure}

Consider a different choice of decomposition $L'$ in place of $L$ above.

This leads to a cycle consisting of filtration degree 1 stuff.

We want to show that this cycle bounds a chain of filtration degree 2 stuff.

Choose a decomposition $M$ which has common refinements with each of 

$K$, $KL$, $L$, $K'L$, $K'$, $K'L'$, $L'$ and $KL'$.

(We also also require that $KLM$ antirefines to $KM$, etc.)

Then we have a filtration degree 2 chain, as shown in Figure \ref{zzz5}, which does the trick.

(Each small triangle in Figure \ref{zzz5} can be filled with a filtration degree 2 chain.)

\begin{figure}[!ht]

%\begin{equation*}

%\mathfig{1.0}{tempkw/zz5}

%\end{equation*}

\begin{equation*}

\begin{tikzpicture}

\node(M) at (0,0) {$M$};

\foreach \angle/\label in {0/K', 45/K'L, 90/L, 135/KL, 180/K, 225/KL', 270/L', 315/K'L'} {

	\node(\label) at (\angle:4) {$\label$};

}

\foreach \label in {K', L, K, L'} {

	\node(\label M) at ($(M)!0.6!(\label)$) {$\label M$};

	\draw[->] (\label M)--(M);

	\draw[->] (\label M)--(\label);

}

\foreach \k in {K, K'} {

	\foreach \l in {L, L'} {

		\node(\k \l M) at (intersection cs: first line={(\k M)--(\l)}, second line={(\l M)--(\k)}) {$\k \l M$};

		\draw[->] (\k \l M)--(M);

		\draw[->] (\k \l M)--(\k \l );

		\draw[->] (\k \l M)--(\k M);

		\draw[->] (\k \l M)--(\l);

		\draw[->] (\k \l M)--(\l M);

		\draw[->] (\k \l M)--(\k);

	}

}

\draw[->] (K'L') to[bend right=10] (K');

\draw[->] (K'L') to[bend left=10] (L');

\draw[->] (KL') to[bend left=10] (K);

\draw[->] (KL') to[bend right=10] (L');

\draw[->] (K'L) to[bend left=10] (K');

\draw[->] (K'L) to[bend right=10] (L);

\draw[->] (KL) to[bend right=10] (K);

\draw[->] (KL) to[bend left=10] (L);

\end{tikzpicture}

\end{equation*}

\caption{Filling in $K$-$KL$-$L$-$K'L$-$K'$-$K'L'$-$L'$-$KL'$-$K$}

\label{zzz5}

\end{figure}

Continuing in this way we see that $D(a)$ is acyclic.

\end{proof}

We are now in a position to apply the method of acyclic models to get a map

$\phi:G_* \to \bc_*^\cF(Y)$.

We may assume that $\phi(a)$ has the form $(a, K) + r$, where $(a, K)$ is in filtration degree zero

and $r$ has filtration degree greater than zero.

We now show that $\phi\circ\psi$ and $\psi\circ\phi$ are homotopic to the identity.

$\psi\circ\phi$ is the identity on the nose:

\[

	\psi(\phi(a)) = \psi((a,K)) + \psi(r) = a + 0.

\]

Roughly speaking, $(a, K)$ is just $a$ chopped up into little pieces, and 

$\psi$ glues those pieces back together, yielding $a$.

We have $\psi(r) = 0$ since $\psi$ is zero in positive filtration degrees.

$\phi\circ\psi$ is the identity up to homotopy by another MoAM argument.

To each generator $(b, \ol{K})$ of $G_*$ we associate the acyclic subcomplex $D(b)$ defined above.

Both the identity map and $\phi\circ\psi$ are compatible with this

collection of acyclic subcomplexes, so by the usual MoAM argument these two maps

are homotopic.

This concludes the proof of Theorem \ref{product_thm}.

\end{proof}

\nn{need to say something about dim $< n$ above}

\medskip

\begin{cor}

\label{cor:new-old}

The new-fangled and old-fashioned blob complexes are homotopic.

\end{cor}

\begin{proof}

Apply Theorem \ref{product_thm} with the fiber $F$ equal to a point.

\end{proof}

\medskip

Theorem \ref{product_thm} extends to the case of general fiber bundles

\[

	F \to E \to Y .

\]

We can generalize the definition of a $k$-category by replacing the categories

of $j$-balls ($j\le k$) with categories of $j$-balls $D$ equipped with a map $p:D\to Y$.

\nn{need citation to other work that does this; Stolz and Teichner?}

Call this a $k$-category over $Y$.

A fiber bundle $F\to E\to Y$ gives an example of a $k$-category over $Y$:

assign to $p:D\to Y$ the blob complex $\bc_*(p^*(E))$.

Let $\cF_E$ denote this $k$-category over $Y$.

We can adapt the homotopy colimit construction (based decompositions of $Y$ into balls) to

get a chain complex $\cF_E(Y)$.

The proof of Theorem \ref{product_thm} goes through essentially unchanged 

to show that

\[

	\bc_*(E) \simeq \cF_E(Y) .

\]

\nn{The second approach: Choose a decomposition $Y = \cup X_i$

such that the restriction of $E$ to $X_i$ is a product $F\times X_i$.

Choose the product structure as well.

To each codim-1 face $D_i\cap D_j$ we have a bimodule ($S^0$-module).

And more generally to each codim-$j$ face we have an $S^{j-1}$-module.

Decorate the decomposition with these modules and do the colimit.

}

\nn{There is a version of this last construction for arbitrary maps $E \to Y$

(not necessarily a fibration).}

\subsection{A gluing theorem}

\label{sec:gluing}

Next we prove a gluing theorem.

Let $X$ be a closed $k$-manifold with a splitting $X = X'_1\cup_Y X'_2$.

We will need an explicit collar on $Y$, so rewrite this as

$X = X_1\cup (Y\times J) \cup X_2$.

Given this data we have: \nn{need refs to above for these}

\begin{itemize}

\item An $A_\infty$ $n{-}k$-category $\bc(X)$, which assigns to an $m$-ball

$D$ fields on $D\times X$ (for $m+k < n$) or the blob complex $\bc_*(D\times X; c)$

(for $m+k = n$). \nn{need to explain $c$}.

\item An $A_\infty$ $n{-}k{+}1$-category $\bc(Y)$, defined similarly.

\item Two $\bc(Y)$ modules $\bc(X_1)$ and $\bc(X_2)$, which assign to a marked

$m$-ball $(D, H)$ either fields on $(D\times Y) \cup (H\times X_i)$ (if $m+k < n$)

or the blob complex $\bc_*((D\times Y) \cup (H\times X_i))$ (if $m+k = n$).

\end{itemize}

\begin{thm}

\label{thm:gluing}

$\bc(X) \cong \bc(X_1) \otimes_{\bc(Y), J} \bc(X_2)$.

\end{thm}

\begin{proof}

The proof is similar to that of Theorem \ref{product_thm}.

\nn{need to say something about dimensions less than $n$, 

but for now concentrate on top dimension.}

Let $\cT$ denote the $n{-}k$-category $\bc(X_1) \otimes_{\bc(Y), J} \bc(X_2)$.

Let $D$ be an $n{-}k$-ball.

There is an obvious map from $\cT(D)$ to $\bc_*(D\times X)$.

To get a map in the other direction, we replace $\bc_*(D\times X)$ with a subcomplex

$\cS_*$ which is adapted to a fine open cover of $D\times X$.

For sufficiently small $j$ (depending on the cover), we can find, for each $j$-blob diagram $b$

on $D\times X$, a decomposition of $J$ such that $b$ splits on the corresponding

decomposition of $D\times X$.

The proof that these two maps are inverse to each other is the same as in

Theorem \ref{product_thm}.

\end{proof}

This establishes Property \ref{property:gluing}.

\medskip

\subsection{Reconstructing mapping spaces}

The next theorem shows how to reconstruct a mapping space from local data.

Let $T$ be a topological space, let $M$ be an $n$-manifold, 

and recall the $A_\infty$ $n$-category $\pi^\infty_{\leq n}(T)$ 

of Example \ref{ex:chains-of-maps-to-a-space}.

Think of $\pi^\infty_{\leq n}(T)$ as encoding everything you would ever

want to know about spaces of maps of $k$-balls into $T$ ($k\le n$).

To simplify notation, let $\cT = \pi^\infty_{\leq n}(T)$.

\begin{thm} \label{thm:map-recon}

The blob complex for $M$ with coefficients in the fundamental $A_\infty$ $n$-category for $T$ is quasi-isomorphic to singular chains on maps from $M$ to $T$.

$$\cB^\cT(M) \simeq C_*(\Maps(M\to T)).$$

\end{thm}

\begin{rem}

\nn{This just isn't true, Lurie doesn't do this! I just heard this from Ricardo...}

Lurie has shown in \cite{0911.0018} that the topological chiral homology of an $n$-manifold $M$ with coefficients in \nn{a certain $E_n$ algebra constructed from $T$} recovers  the same space of singular chains on maps from $M$ to $T$, with the additional hypothesis that $T$ is $n-1$-connected. This extra hypothesis is not surprising, in view of the idea that an $E_n$ algebra is roughly equivalent data as an $A_\infty$ $n$-category which is trivial at all but the topmost level.

\end{rem}

\begin{proof}

We begin by constructing chain map $g: \cB^\cT(M) \to C_*(\Maps(M\to T))$.

We then use \ref{extension_lemma_b} to show that $g$ induces isomorphisms on homology.

Recall that the homotopy colimit $\cB^\cT(M)$ is constructed out of a series of

$j$-fold mapping cylinders, $j \ge 0$.

So, as an abelian group (but not as a chain complex), 

\[

	\cB^\cT(M) = \bigoplus_{j\ge 0} C^j,

\]

where $C^j$ denotes the new chains introduced by the $j$-fold mapping cylinders.

Recall that $C^0$ is a direct sum of chain complexes with the summands indexed by

decompositions of $M$ which have their $n{-}1$-skeletons labeled by $n{-}1$-morphisms

of $\cT$.

Since $\cT = \pi^\infty_{\leq n}(T)$, this means that the summands are indexed by pairs

$(K, \vphi)$, where $K$ is a decomposition of $M$ and $\vphi$ is a continuous

maps from the $n{-}1$-skeleton of $K$ to $T$.

The summand indexed by $(K, \vphi)$ is

\[

	\bigotimes_b D_*(b, \vphi),

\]

where $b$ runs through the $n$-cells of $K$ and $D_*(b, \vphi)$ denotes

chains of maps from $b$ to $T$ compatible with $\vphi$.

We can take the product of these chains of maps to get a chains of maps from

all of $M$ to $K$.

This defines $g$ on $C^0$.

We define $g(C^j) = 0$ for $j > 0$.

It is not hard to see that this defines a chain map from 

$\cB^\cT(M)$ to $C_*(\Maps(M\to T))$.

%%%%%%%%%%%%%%%%%

\noop{

Next we show that $g$ induces a surjection on homology.

Fix $k > 0$ and choose an open cover $\cU$ of $M$ fine enough so that the union 

of any $k$ open sets of $\cU$ is contained in a disjoint union of balls in $M$.

\nn{maybe should refer to elsewhere in this paper where we made a very similar argument}

Let $S_*$ be the subcomplex of $C_*(\Maps(M\to T))$ generated by chains adapted to $\cU$.

It follows from Lemma \ref{extension_lemma_b} that $C_*(\Maps(M\to T))$

retracts onto $S_*$.

Let $S_{\le k}$ denote the chains of $S_*$ of degree less than or equal to $k$.

We claim that $S_{\le k}$ lies in the image of $g$.

Let $c$ be a generator of $S_{\le k}$ --- that is, a $j$-parameter family of maps $M\to T$,

$j \le k$.

We chose $\cU$ fine enough so that the support of $c$ is contained in a disjoint union of balls

in $M$.

It follow that we can choose a decomposition $K$ of $M$ so that the support of $c$ is 

disjoint from the $n{-}1$-skeleton of $K$.

It is now easy to see that $c$ is in the image of $g$.

Next we show that $g$ is injective on homology.

}

\nn{...}

\end{proof}

\nn{maybe should also mention version where we enrich over

spaces rather than chain complexes; should comment on Lurie's \cite{0911.0018} (and others') similar result

for the $E_\infty$ case, and mention that our version does not require 

any connectivity assumptions}

\medskip

\hrule

\medskip

\nn{to be continued...}

\medskip

\nn{still to do: fiber bundles, general maps}

\todo{}

Various citations we might want to make:

\begin{itemize}

\item \cite{MR2061854} McClure and Smith's review article

\item \cite{MR0420610} May, (inter alia, definition of $E_\infty$ operad)

\item \cite{MR0236922,MR0420609} Boardman and Vogt

\item \cite{MR1256989} definition of framed little-discs operad

\end{itemize}

We now turn to establishing the gluing formula for blob homology, restated from Property \ref{property:gluing} in the Introduction

\begin{itemize}

%\mbox{}% <-- gets the indenting right

\item For any $(n-1)$-manifold $Y$, the blob homology of $Y \times I$ is

naturally an $A_\infty$ category. % We'll write $\bc_*(Y)$ for $\bc_*(Y \times I)$ below.

\item For any $n$-manifold $X$, with $Y$ a codimension $0$-submanifold of its boundary, the blob homology of $X$ is naturally an

$A_\infty$ module for $\bc_*(Y \times I)$.

\item For any $n$-manifold $X$, with $Y \cup Y^{\text{op}}$ a codimension

$0$-submanifold of its boundary, the blob homology of $X'$, obtained from

$X$ by gluing along $Y$, is the $A_\infty$ self-tensor product of

$\bc_*(X)$ as an $\bc_*(Y \times I)$-bimodule.

\begin{equation*}

\bc_*(X') \iso \bc_*(X) \Tensor^{A_\infty}_{\mathclap{\bc_*(Y \times I)}} \!\!\!\!\!\!\xymatrix{ \ar@(ru,rd)@<-1ex>[]}

\end{equation*}

\end{itemize}