Talk:Extension problem

I think that the distinction between K being a subgroup of G or only being isomorphic to one is not important, because there is a specific isomorphism between them. This language doesn't really generalize anything, it just makes it a bit more confusing. Gregory Muller 13:58, 11 September 2006 (UTC)[reply]

Merge with Group Extension

I'm not sure there are enough interesting things to say about group extensions other than their classification to merit its own entry. Gregory Muller 13:58, 11 September 2006 (UTC)[reply]

Mistake

The claim that $Ext^{1}(H,K)$ classifies central extensions of $H$ by $K$ is wrong. $Ext^{1}$ classifies only abelian extensions. Central extensions are classified by group cohomology, namely $H^{2}(H,K)$ , which is a gadget that makes sense after we've specified an action of $H$ on $K$ . So, someone should fix this. It may have to be me. John Baez 18:32, 26 January 2007 (UTC)[reply]