## Illinois Journal of Mathematics

- Illinois J. Math.
- Volume 48, Number 1 (2004), 171-197.

### Dickson invariants, regularity and computation in group cohomology

#### Abstract

In this paper, we investigate the commutative algebra of the cohomology ring $H^*(G,k)$ of a finite group $G$ over a field $k$. We relate the concept of quasi-regular sequence, introduced by Benson and Carlson, to the local cohomology of the cohomology ring. We give some slightly strengthened versions of quasi-regularity, and relate one of them to Castelnuovo--Mumford regularity. We prove that the existence of a quasi-regular sequence in either the original sense or the strengthened ones is true if and only if the Dickson invariants form a quasi-regular sequence in the same sense. The proof involves the notion of virtual projectivity, introduced by Carlson, Peng and Wheeler.

As a by-product of this investigation, we give a new proof of the Bourguiba--Zarati theorem on depth and Dickson invariants, in the context of finite group cohomology, without using the machinery of unstable modules over the Steenrod algebra.

Finally, we describe an improvement of Carlson's algorithm for computing the cohomology of a finite group using a finite initial segment of a projective resolution of the trivial module. In contrast to Carlson's algorithm, ours does not depend on verifying any conjectures during the course of the calculation, and is always guaranteed to work.

#### Article information

**Source**

Illinois J. Math., Volume 48, Number 1 (2004), 171-197.

**Dates**

First available in Project Euclid: 13 November 2009

**Permanent link to this document**

https://projecteuclid.org/euclid.ijm/1258136180

**Digital Object Identifier**

doi:10.1215/ijm/1258136180

**Mathematical Reviews number (MathSciNet)**

MR2048221

**Zentralblatt MATH identifier**

1041.20036

**Subjects**

Primary: 20J06: Cohomology of groups

Secondary: 13A50: Actions of groups on commutative rings; invariant theory [See also 14L24] 13D45: Local cohomology [See also 14B15]

#### Citation

Benson, Dave. Dickson invariants, regularity and computation in group cohomology. Illinois J. Math. 48 (2004), no. 1, 171--197. doi:10.1215/ijm/1258136180. https://projecteuclid.org/euclid.ijm/1258136180