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Varieties of idempotent groupoids with small clones J. Gałuszka (1999) Colloquium Mathematicae We give an equational description of all idempotent groupoids with at most three essentially n-ary term operations.
Varieties of operator semigroups representing partitions. R. Padmanabhan, H. Lakser, B. Wolk (1995) Semigroup forum