cohomology

# Contents

## Idea

Where cohomology classifies cocycles on an object $X$ with coefficients in some object $A$, relative cohomology for a map morphism $Y \to X$ classifies cocycles on $X$ that satisfy some condition when pulled back to $Y$, such as being trivializable there. Or rather: cocycles equipped with some structure on their pullback to $Y$, such as a trivialization.

## Definition

We first give a general abstract definition and then reduce to certain special cases.

### General definition

Recall the general abstract definition of cohomology, as discussed there:

for $\mathbf{H}$ an (∞,1)-topos, and $X, A \in \mathbf{H}$ two objects, a cocycle on $X$ with coefficients in $A$ is a morphism $X \to A$, the ∞-groupoid of cocycles, coboundaries and higher coboundaries is the (∞,1)-categorical hom space $\mathbf{H}(X,A)$ and the cohomology classes themselves are the connected components / homotopy classes in there

$H(X,A) := \pi_0 \mathbf{H}(X,A) \,.$
###### Definition

Let $i : Y \to X$ and $f : B \to A$ be two morphisms in $\mathbf{H}$. Then the relative cohomology of $X$ with coefficients in $A$ relative to these morphisms is the connected components of the $\infty$-groupoid of relative cocycles

$H_{Y}^B(X,A) := \pi_0 \mathbf{H}^I(Y \stackrel{i}{\to} X\;,\; B \stackrel{f}{\to} A) \,,$

where $\mathbf{H}^I$ the arrow (∞,1)-topos of $\mathbf{H}$, hence the (∞,1)-category of (∞,1)-functors $I \to \mathbf{H}$, where $I$ is the interval category.

###### Remark

Often relative cohomology is considered for the special case where $A$ is a pointed object, $B = *$ is the terminal object and $B \simeq * \to A$ is the point inclusion. In this case we may write just

$H_Y(X,A) := H_Y^*(X,A) \,.$
###### Remark

The $\infty$-groupoid of relative cocycles is the (∞,1)-pullback in

$\array{ \mathbf{H}^I(Y \to X, B \to A) &\to& \mathbf{H}(Y, B) \\ \downarrow && \downarrow^{\mathrlap{f_*}} \\ \mathbf{H}(X, A) &\stackrel{i^*}{\to}& \mathbf{H}(Y,A) } \,.$

This makes manifest the interpretation of relative cocycles as $A$-cocycles on $X$ whose restriction to $Y$ is equipped with a coboundary to a $B$-cocycle on $Y$.

If $B$ is the point then this means: $A$ cocycles on $X$ which are equipped with a trivialization on $Y$.

###### Remark

If we fix an $A$-cocycle $\mathbf{c}$ on $X$, then we may consider the relative cohomology just of this cocycle, hence the homotopy fiber of the $\infty$-groupoid of relative cocycles over $\mathbf{c}$

$\array{ \mathbf{H}^I_{\mathbf{c}}(i,f) &\to& \mathbf{H}^I(i,f) \\ \downarrow && \downarrow \\ * &\stackrel{\mathbf{c}}{\to}& \mathbf{H}(X,A) } \,.$

By the pasting law it follows that this fiber is equivalently given by the following (∞,1)-pullback

$\array{ \mathbf{H}^I_{\mathbf{c}}(i,f) &\to& \mathbf{H}(Y,B) \\ \downarrow && \downarrow \\ * &\stackrel{i^* \mathbf{c}}{\to}& \mathbf{H}(Y,A) } \,.$

Comparison shows that this identifies $\mathbf{H}^I_{\mathbf{c}}(i,f)$ as the cocycle $\infty$-groupoid of the $[i^* \mathbf{c}]$-twisted cohomology of $Y$ with coefficients in the homotopy fiber of $f$.

See for instance the example of twisted bundles on D-branes below.

### In chain complexes

A special case of the general definition of cohomology is abelian sheaf cohomology, obtained by taking the coefficient objects $A$ and $B$ to be in the image of chain complexes under the Dold-Kan correspondence.

If moreover we restrict attention to the case that $B = *$, then by remark 1 the relative cohomology is given by the homotopy fiber of a morphism of cochain complexes $C^\bullet(X,A) \to C^\bullet(Y,A)$, presenting the morphism $\mathbf{H}(X, A) \to \mathbf{H}(Y,A)$. Such homotopy fibers are given for instance by dual mapping cone complexes. Accordingly, in the abelian case the $A$-cohomology on $X$ relative to $Y$ is the cochain cohomology of this mapping cone complex. This is the definition one finds in much traditional literature.

## Examples

### Twisted bundles on D-branes

We consider an example of relative cohomology for the general case where $B \to A$ is not the point inclusion, but exhibits an additional twist, according to 2.

This example is motivated from the physics of D-branes in type II string theory, as well as from twisted K-theory.

Let the ambient (∞,1)-topos be $\mathbf{H} :=$ Smooth∞Grpd. For any $n \in \mathbb{N}$ consider the sequence of Lie groups

$U(1) \to U(n) \to PU(n) \,,$

exhibiting the unitary group as a central extension of groups of the projective unitary group. The fiber sequence on delooping smooth ∞-groupoids induced by this is

$\mathbf{B}U(1) \to \mathbf{B}U(n) \to \mathbf{B} PU(n) \stackrel{f}{\to} \mathbf{B}^2 U(1) \,.$

For $Y$ a smooth manifold, the $f$-twisted cohomology of $X$ is classifies $U(n)$-principal twisted bundles on $Y$, as discussed there. These appear in string theory/twisted K-theory, where the twist is a B-field/circle 2-bundle/bundle gerbe classified by $Y \to \mathbf{B}^2 U(1)$.

But in these applications, this 2-bundle is the restriction of an ambient 2-bundle classified by $\mathbf{c} : X \to \mathbf{B}^2 U(1)$ on a spacetime $X$ along an embedding $Y \hookrightarrow X$ of the D-brane worldvolume. Therefore the moduli stack of total field configurations consisting of the ambient B-field and the gauge field on the D-brane (see Freed-Witten anomaly for the details of this mechanism) is the $\infty$-groupoid of $(Y \hookrightarrow X)$-relative cocycles with coefficients in $(\mathbf{B}PU \to \mathbf{B}^2 U(1))$.

Revised on January 18, 2013 03:52:11 by Urs Schreiber (203.116.137.162)