On the second cohomology of semidirect products

Sebastien Leroy

Description

On the second cohomology of semidirect products Manfred Hartl and Sebastien Leroy June 20, 2007 LAMAV, ISTV2, Universite de Valenciennes et du Hainaut-Cambresis, Le Mont Houy, 59313 Valenciennes Cedex 9, France. Email: Phone no 0033/327511901, Fax [phone]. Abstract Let G be a group which is the semidirect product of a normal sub- group N and a subgroup T , and let M be a G-module with not neces- sarily trivial G-action. Then we embed the simultaneous restriction map res = (resGN , res G T ) t : H2(G,M) ? H2(N,M)T ?H2(T,M) into a natural five term exact sequence consisting of one and two-dimensional cohomology groups of the factors N and T . The elements of H2(G,M) are represented in terms of group extensions of G by M constructed from extensions of N and T . Introduction. The low dimensional cohomology groups Hn(G,M), n ≤ 2, of a group G with coefficients in a G-module M crucially occur in many fields, in algebra as well as in geometry. In fact, they reflect the structure of G (and of M if the G-action on it is non trivial) in a subtle way which is far from being understood in general.

• pi ??


• now ready


• sequence


• auto- morphism groups


• group extension


• universite de valenciennes et du hainaut cambresis


• normal subgroup


• short exact

Sujets

• Tahara
• Group extension
• Normal subgroup

Informations

• Publié par: pefav
• Nombre de lectures: 18
• Langue: English

Extrait

On the second cohomology of semidirect products ManfredHartlandSe´bastienLeroy June 20, 2007

LAMAV, ISTV2, Universit´edeValenciennesetduHainaut-Cambr´esis, Le Mont Houy, 59313 Valenciennes Cedex 9, France. Email: [e-mail] Phone n o 0033/327511901, Fax [phone].

Abstract Let G be a group which is the semidirect product of a normal sub- group N and a subgroup T , and let M be a G -module with not neces- sarily trivial G -action. Then we embed the simultaneous restriction map res = ( res G N , res TG ) t : H 2 ( G, M ) → H 2 ( N, M ) T × H 2 ( T , M ) into a natural five term exact sequence consisting of one and two-dimensional cohomology groups of the factors N and T . The elements of H 2 ( G, M ) are represented in terms of group extensions of G by M constructed from extensions of N and T .

Introduction. The low dimensional cohomology groups H n ( G M ) , n ≤ 2 , of a group G with coefficients in a G -module M crucially occur in many fields, in algebra as well as in geometry. In fact, they reflect the structure of G (and of M if the G -action on it is non trivial) in a subtle way which is far from being understood in general. If G admits a proper normal subgroup N it can be viewed as an extension 1 → N → G → Q → 1  (1) and one wishes to express the cohomology of G in terms of the cohomology of the simpler “pieces” N and Q . Formally, the Lyndon-Hochschild-Serre spectral sequence (referred to as LHSSS in the sequel) H p ( Q H n − p ( N M )) ⇒ H n ( G M ) solves this problem, computing certain filtration quotients of H n ( G M ) provided one can manage to compute the corresponding differentials; those concerning

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