Planarity ranks of modular varieties of semigroups

Abstract

By the planarity rank of a semigroup variety we mean the largest number of generators of a free semigroup of a variety with respect to which the semigroup admits a planar Cayley graph. Since the time when L.M.Martynov formulated the problem of describing the planarity ranks of semigroup varieties, many specific results have been obtained in this direction. A modular variety of semigroups is a variety of semigroups with a modular lattice of subvarieties. In this paper, we calculate the exact values of the planarity ranks of an infinite countable set of all possible modular varieties of semigroups. It turns out that these values do not exceed 3. Machine calculations are mostly used in the proof. Prover9 and Mace4 are used to check the equalities of elements of free semigroups of varieties defined by a large number of identities. To prove the non-planarity of graphs, the Pontryagin-Kuratovsky criterion is used, and the Colin de Verdiere invariant is indirectly used to justify planarity.