A partial generalization of a theorem of Hursch Charles J. Mozzochi (1968) Commentationes Mathematicae Universitatis Carolinae
Concerning minimal primitive classes of algebras containing any category of algebras as a full subcategory Jiří Sichler (1968) Commentationes Mathematicae Universitatis Carolinae
Primitive classes of algebras with two unary idempotent operations, containing all algebraic categories as full subcategories Aleš Pultr, Jiří Sichler (1969) Commentationes Mathematicae Universitatis Carolinae