Conjecture on maximal finite subgroups of cactus groups

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Let JnJ_n be the cactus group on nn strands. A finite group embeds into JnJ_n precisely when it embeds into an iterated permutational wreath product of copies of Z/2Z\mathbb{Z}/2\mathbb{Z} of the form

(⋯(Z/2Z≀r1Z/2Z)≀r2⋯ )≀rsZ/2Z,(\cdots (\mathbb{Z}/2\mathbb{Z} \wr_{r_1} \mathbb{Z}/2\mathbb{Z}) \wr_{r_2} \cdots ) \wr_{r_s} \mathbb{Z}/2\mathbb{Z},

where r1,…,rsr_1,\ldots,r_s satisfy r1⋯rs≤n/2r_1\cdots r_s\leq n/2. Here, for a group AA and r≥1r\geq 1, A≀rZ/2ZA\wr_r\mathbb{Z}/2\mathbb{Z} denotes Ar⋊Z/2ZA^r\rtimes\mathbb{Z}/2\mathbb{Z}, with the right factor reversing the rr factors. Maximal finite subgroup conjecture. A finite group embeds into the cactus group JnJ_n if and only if it embeds into

(⋯(Z/2Z≀r1Z/2Z)≀r2⋯ )≀rsZ/2Z(\cdots (\mathbb{Z}/2\mathbb{Z} \wr_{r_1} \mathbb{Z}/2\mathbb{Z}) \wr_{r_2} \cdots ) \wr_{r_s} \mathbb{Z}/2\mathbb{Z}

for some r1,…,rsr_1,\ldots,r_s satisfying r1⋯rs≤n/2r_1\cdots r_s\leq n/2. This would give an explicit algebraic description of finite subgroups, beyond the fact that they are 22-groups, using the median geometry of cactus groups.

References

Primary source

Anthony Genevois, “Cactus groups from the viewpoint of geometric group theory”, arXiv:2212.03494 (2022).

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