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Published online by Cambridge University Press: 25 November 2024
We study the planar 3-colorablesubgroup $\mathcal{E}$ of Thompson’s group F and its even part
${\mathcal{E}_{\rm EVEN}}$. The latter is obtained by cutting
$\mathcal{E}$ with a finite index subgroup of F isomorphic to F, namely the rectangular subgroup
$K_{(2,2)}$. We show that the even part
${\mathcal{E}_{\rm EVEN}}$ of the planar 3-colorable subgroup admits a description in terms of stabilisers of suitable subsets of dyadic rationals. As a consequence
${\mathcal{E}_{\rm EVEN}}$ is closed in the sense of Golan and Sapir. We then study three quasi-regular representations associated with
${\mathcal{E}_{\rm EVEN}}$: two are shown to be irreducible and one to be reducible.