Sebastià Mijares ; Enric Ventura - Onto extensions of free groups

gcc:7036 - journal of Groups, complexity, cryptology, April 15, 2021, Volume 13, issue 1 - https://doi.org/10.46298/jgcc.2021.13.1.7036
Onto extensions of free groups

Authors: Sebastià Mijares ; Enric Ventura

An extension of subgroups $H\leqslant K\leqslant F_A$ of the free group of rank $|A|=r\geqslant 2$ is called onto when, for every ambient free basis $A'$, the Stallings graph $\Gamma_{A'}(K)$ is a quotient of $\Gamma_{A'}(H)$. Algebraic extensions are onto and the converse implication was conjectured by Miasnikov-Ventura-Weil, and resolved in the negative, first by Parzanchevski-Puder for rank $r=2$, and recently by Kolodner for general rank. In this note we study properties of this new type of extension among free groups (as well as the fully onto variant), and investigate their corresponding closure operators. Interestingly, the natural attempt for a dual notion -- into extensions -- becomes trivial, making a Takahasi type theorem not possible in this setting.


Volume: Volume 13, issue 1
Published on: April 15, 2021
Submitted on: January 2, 2021
Keywords: Mathematics - Group Theory,20E05, 20E07


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