On commutative set-theoretic solutions of the Pentagon Equation
摘要
We provide a notion of retract relation for commutative non-degenerate set-theoretic solutions of the Pentagon Equation, which is consistent with the one given by Colazzo, Jespers, and Kubat (Set-theoretic solutions of the pentagon equation, Commun. Math. Phys. 380(2), 1003–1024 (2020)) for involutive solutions. Moreover, we develop a machinery involving a permutation group, which we will call associated permutation group, useful to construct all these solutions and we will provide a description of the irretractable ones. Finally, non-degenerate solutions on left-zero semigroup are studied in detail, with an emphasis on the ones with cyclic associated permutation group and on the ones having small size.