Semigroups with idempotent powers, II: The lattices of equational subclasses of [(xy2 =xy] . J. A. Gerhard (1977) Semigroup forum
Some examples of primitive lattices Jaroslav Ježek, Václav Slavík (1973) Acta Universitatis Carolinae. Mathematica et Physica
Symmetry classes of tensors via semigroup operands. Isidore Fleischer (1986) Aequationes mathematicae