G Birkhoff 1935: a class of algebras of given signature is a variety iff closed under H (homomorphic images), S (subalgebras), P (products); foundation of equational-logic + UA.
G Birkhoff 1935: a class of algebras of given signature is a variety iff closed under H (homomorphic images), S (subalgebras), P (products); foundation of equational-logic + UA.