Conjecture on maximal finite subgroups of cactus groups

From papers

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/2Zr1Z/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 r1rsn/2r_1\cdots r_s\leq n/2. Here, for a group AA and r1r\geq 1, ArZ/2ZA\wr_r\mathbb{Z}/2\mathbb{Z} denotes ArZ/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/2Zr1Z/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 r1rsn/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.

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Sources & referencesView supporting material

Primary source

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

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