Modular elements in congruence lattices of -sets.
On modular elements of the lattice of semigroup varieties
A semigroup variety is called if it is a modular element of the lattice of all semigroup varieties. We obtain a strong necessary condition for a semigroup variety to be modular. In particular, we prove that every modular nil-variety may be given by 0-reduced identities and substitutive identities only. (An identity is called if the words and depend on the same letters and may be obtained from by renaming of letters.) We completely determine all commutative modular varieties and obtain...
On congruences of -sets
We describe -sets whose congruences satisfy some natural lattice or multiplicative restrictions. In particular, we determine -sets with distributive, arguesian, modular, upper or lower semimodular congruence lattice as well as congruence -permutable -sets for .
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