Malnormal Subgroups of Finitely Presented Groups

Abstract: The following refinement of the Higman embedding theorem is proved: Given a finitely generated recursively presented group $R$, there exists a quasi-isometric malnormal embedding of $R$ into a finitely presented group $H$ such that the image of the embedding enjoys the Congruence Extension Property. Moreover, it is shown that the group $H$ can be constructed to have decidable Word problem if and only if the Word problem of $R$ is decidable, yielding a refinement of a theorem of Clapham. Finally, it is proved that for any countable group $G$ and any computable function $\ell:G\to\mathbb{N}$ satisfying some necessary requirements, there exists a malnormal embedding enjoying the Congruence Extension Property of $G$ into a finitely presented group $H$ such that the restriction of $|\cdot|_H$ to $G$ is equivalent to $\ell$, producing a refinement of a result of Ol'shanskii.