Idempotents in The Groupoid of all SP Classes of Lattices
Alan Day · Canadian Mathematical Bulletin · 1978
In [5], Mal'cev generalized the group theoretical results of H. Neumann (see [6] Chapter 2) to produce the notion of the product, of two subclasses of a given variety of algebras, Following the group theorectic example, members of were called extensions of algebras in by algebras in