# Lyndon–Hochschild–Serre spectral sequence

In mathematics, especially in the fields of group cohomology, homological algebra and number theory the Lyndon spectral sequence or Hochschild–Serre spectral sequence is a spectral sequence relating the group cohomology of a normal subgroup N and the quotient group G/N to the cohomology of the total group G.

## Statement

The precise statement is as follows:

Let G be a group and N be a normal subgroup. The latter ensures that the quotient G/N is a group, as well. Finally, let A be a G-module. Then there is a spectral sequence of cohomological type

H p(G/N, H q(N, A)) ⇒ H p+q(G, A)

and there is a spectral sequence of homological type

H p(G/N, H q(N, A)) ⇒ H p+q(G, A).

The same statement holds if G is a profinite group, N is a closed normal subgroup and H* denotes the continuous cohomology.

### Example: Cohomology of the Heisenberg group

The spectral sequence can be used to compute the homology of the Heisenberg group G with integral entries, i.e., matrices of the form

${\displaystyle \left({\begin{array}{ccc}1&a&b\\0&1&c\\0&0&1\end{array}}\right),\ a,b,c\in \mathbb {Z} .}$

This group is an extension

${\displaystyle 0\to \mathbb {Z} \to G\to \mathbb {Z} \oplus \mathbb {Z} \to 0}$

corresponding to the subgroup with a=c=0. The spectral sequence for the group homology, together with the analysis of a differential in this spectral sequence, shows that[1]

${\displaystyle H_{i}(G,\mathbb {Z} )=\left\{{\begin{array}{cc}\mathbb {Z} &i=0,3\\\mathbb {Z} \oplus \mathbb {Z} &i=1,2\\0&i>3.\end{array}}\right.}$

### Example: Cohomology of wreath products

For a group G, the wreath product is an extension

${\displaystyle 1\to G^{p}\to G\wr \mathbb {Z} /p\to \mathbb {Z} /p\to 1.}$

The resulting spectral sequence of group cohomology with coefficients in a field k,

${\displaystyle H^{r}(\mathbb {Z} /p,H^{s}(G^{p},k))\Rightarrow H^{r+s}(G\wr \mathbb {Z} /p,k),}$

is known to degenerate at the ${\displaystyle E_{2}}$-page.[2]

## Properties

The associated five-term exact sequence is the usual inflation-restriction exact sequence:

0 → H 1(G/N, AN) → H 1(G, A) → H 1(N, A)G/NH 2(G/N, AN) →H 2(G, A).

## Generalizations

The spectral sequence is an instance of the more general Grothendieck spectral sequence of the composition of two derived functors. Indeed, H(G, -) is the derived functor of (−)G (i.e. taking G-invariants) and the composition of the functors (−)N and (−)G/N is exactly (−)G.

A similar spectral sequence exists for group homology, as opposed to group cohomology, as well.[3]

## References

1. ^ Kevin Knudson. Homology of Linear Groups. Birkhäuser. Example A.2.4
2. ^ Minoru Nakaoka (1960), "Decomposition Theorem for Homology Groups of Symmetric Groups", Annals of Mathematics, Second Series, 71 (1): 16–42, JSTOR 1969878, for a brief summary see section 2 of Carlson, Jon F.; Henn, Hans-Werner (1995), "Depth and the cohomology of wreath products", Manuscripta Math., 87 (2): 145–151
3. ^ McCleary, John (2001), A User's Guide to Spectral Sequences, Cambridge Studies in Advanced Mathematics, 58 (2nd ed.), Cambridge University Press, doi:10.2277/0521567599, ISBN 978-0-521-56759-6, MR 1793722, Theorem 8bis.12