Quasivariety = class closed under SP (subalgebras + products) + ultraproducts; Mal cev 1966 + McKenzie 1996: quasi-equational definability via Horn clauses.
Quasivariety = class closed under SP (subalgebras + products) + ultraproducts; Mal cev 1966 + McKenzie 1996: quasi-equational definability via Horn clauses.