ON MODULAR AND CANCELLABLE ELEMENTS OF THE LATTICE OF SEMIGROUP VARIETIES

We continue a study of modular and cancellable elements in the lattice SEM of all semigroup varieties. In 2007, the second author completely determined all commutative semigroup varieties that are modular elements in SEM. In 2018 the authors jointly with S.V.Gusev proved that, within the class of commutative varieties, the properties to be modular and cancellable elements in SEM are equivalent. The objective of this article is to verify that, within some slightly wider class of semigroup varieties, this equivalence is not the case. To achieve this goal, we completely classify semigroup varieties satisfying a permutational identity of length 3 that are modular elements in SEM. Further, we specify a variety with these properties that is not a cancellable element in SEM.