diff -r b15dafe85ee1 -r 77a311b5e2df text/hochschild.tex --- a/text/hochschild.tex Mon Oct 26 17:14:35 2009 +0000 +++ b/text/hochschild.tex Tue Oct 27 02:11:36 2009 +0000 @@ -66,7 +66,7 @@ Next, we show that for any $C$-$C$-bimodule $M$, \begin{prop} \label{prop:hoch} -The complex $K_*(M)$ is quasi-isomorphic to $HC_*(M)$, the usual +The complex $K_*(M)$ is quasi-isomorphic to $\HC_*(M)$, the usual Hochschild complex of $M$. \end{prop} \begin{proof} @@ -74,19 +74,19 @@ up to quasi-isomorphism, by the following properties: \begin{enumerate} \item \label{item:hochschild-additive}% -$HC_*(M_1 \oplus M_2) \cong HC_*(M_1) \oplus HC_*(M_2)$. +$\HC_*(M_1 \oplus M_2) \cong \HC_*(M_1) \oplus \HC_*(M_2)$. \item \label{item:hochschild-exact}% An exact sequence $0 \to M_1 \into M_2 \onto M_3 \to 0$ gives rise to an -exact sequence $0 \to HC_*(M_1) \into HC_*(M_2) \onto HC_*(M_3) \to 0$. +exact sequence $0 \to \HC_*(M_1) \into \HC_*(M_2) \onto \HC_*(M_3) \to 0$. \item \label{item:hochschild-coinvariants}% -$HH_0(M)$ is isomorphic to the coinvariants of $M$, $\coinv(M) = +$\HH_0(M)$ is isomorphic to the coinvariants of $M$, $\coinv(M) = M/\langle cm-mc \rangle$. \item \label{item:hochschild-free}% -$HC_*(C\otimes C)$ is contractible. +$\HC_*(C\otimes C)$ is contractible. (Here $C\otimes C$ denotes the free $C$-$C$-bimodule with one generator.) -That is, $HC_*(C\otimes C)$ is -quasi-isomorphic to its $0$-th homology (which in turn, by \ref{item:hochschild-coinvariants}, is just $C$) via the quotient map $HC_0 \onto HH_0$. +That is, $\HC_*(C\otimes C)$ is +quasi-isomorphic to its $0$-th homology (which in turn, by \ref{item:hochschild-coinvariants}, is just $C$) via the quotient map $\HC_0 \onto \HH_0$. \end{enumerate} (Together, these just say that Hochschild homology is `the derived functor of coinvariants'.) We'll first recall why these properties are characteristic. @@ -110,32 +110,32 @@ \intertext{and} \cP_*(F_j) & \xrightarrow{\cP_0(F_j) \onto H_0(\cP_*(F_j))} \coinv(F_j). \end{align*} -The cone of each chain map is acyclic. In the first case, this is because the `rows' indexed by $i$ are acyclic since $HC_i$ is exact. +The cone of each chain map is acyclic. In the first case, this is because the `rows' indexed by $i$ are acyclic since $\HC_i$ is exact. In the second case, this is because the `columns' indexed by $j$ are acyclic, since $F_j$ is free. Because the cones are acyclic, the chain maps are quasi-isomorphisms. Composing one with the inverse of the other, we obtain the desired quasi-isomorphism $$\cP_*(M) \quismto \coinv(F_*).$$ %If $M$ is free, that is, a direct sum of copies of %$C \tensor C$, then properties \ref{item:hochschild-additive} and -%\ref{item:hochschild-free} determine $HC_*(M)$. Otherwise, choose some +%\ref{item:hochschild-free} determine $\HC_*(M)$. Otherwise, choose some %free cover $F \onto M$, and define $K$ to be this map's kernel. Thus we %have a short exact sequence $0 \to K \into F \onto M \to 0$, and hence a -%short exact sequence of complexes $0 \to HC_*(K) \into HC_*(F) \onto HC_*(M) +%short exact sequence of complexes $0 \to \HC_*(K) \into \HC_*(F) \onto \HC_*(M) %\to 0$. Such a sequence gives a long exact sequence on homology %\begin{equation*} %%\begin{split} -%\cdots \to HH_{i+1}(F) \to HH_{i+1}(M) \to HH_i(K) \to HH_i(F) \to \cdots % \\ -%%\cdots \to HH_1(F) \to HH_1(M) \to HH_0(K) \to HH_0(F) \to HH_0(M). +%\cdots \to \HH_{i+1}(F) \to \HH_{i+1}(M) \to \HH_i(K) \to \HH_i(F) \to \cdots % \\ +%%\cdots \to \HH_1(F) \to \HH_1(M) \to \HH_0(K) \to \HH_0(F) \to \HH_0(M). %%\end{split} %\end{equation*} -%For any $i \geq 1$, $HH_{i+1}(F) = HH_i(F) = 0$, by properties +%For any $i \geq 1$, $\HH_{i+1}(F) = \HH_i(F) = 0$, by properties %\ref{item:hochschild-additive} and \ref{item:hochschild-free}, and so -%$HH_{i+1}(M) \iso HH_i(F)$. For $i=0$, \todo{}. +%$\HH_{i+1}(M) \iso \HH_i(F)$. For $i=0$, \todo{}. % %This tells us how to -%compute every homology group of $HC_*(M)$; we already know $HH_0(M)$ +%compute every homology group of $\HC_*(M)$; we already know $\HH_0(M)$ %(it's just coinvariants, by property \ref{item:hochschild-coinvariants}), -%and higher homology groups are determined by lower ones in $HC_*(K)$, and +%and higher homology groups are determined by lower ones in $\HC_*(K)$, and %hence recursively as coinvariants of some other bimodule. Proposition \ref{prop:hoch} then follows from the following lemmas, establishing that $K_*$ has precisely these required properties. @@ -390,7 +390,7 @@ \medskip For purposes of illustration, we describe an explicit chain map -$HC_*(M) \to K_*(M)$ +$\HC_*(M) \to K_*(M)$ between the Hochschild complex and the blob complex (with bimodule point) for degree $\le 2$. This map can be completed to a homotopy equivalence, though we will not prove that here. @@ -398,7 +398,7 @@ Describing the extension to higher degrees is straightforward but tedious. \nn{but probably we should include the general case in a future version of this paper} -Recall that in low degrees $HC_*(M)$ is +Recall that in low degrees $\HC_*(M)$ is \[ \cdots \stackrel{\bd}{\to} M \otimes C\otimes C \stackrel{\bd}{\to} M \otimes C \stackrel{\bd}{\to} M