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Questions about the branch of algebra that deals with groups.

11 votes
3 answers
1k views

Centralizers in amalgamated free products

An old result of Karrass and Solitar from 1970 says that if $g$ is a nontrivial element in an amalgamated free product $G=A*_HB$, with $H$ malnormal in $G$, then the centralizer of $g$ in $G$ is eithe …
bemihai's user avatar
  • 165
4 votes
0 answers
355 views

Centralizers of elements in HNN extensions

Let $G$ be a (countable) group, $H<G$ be a proper subgroup and $\theta:H\to G$ be an injective group homomorphism such that $\theta(H)\neq G$. Consider the HNN extension $\Gamma=<G,t \mid tHt^{-1}=\th …
bemihai's user avatar
  • 165
1 vote
1 answer
229 views

Relative amenability of subgroups

Let $\Gamma$ be a countable group and let $\Lambda_1,\Lambda_2<\Gamma$ be subgroups. We say that $\Lambda_1$ is amenable relative to $\Lambda_2$ if the action of $\Lambda_1$ on $\Gamma/\Lambda_2$ by l …
bemihai's user avatar
  • 165