Birkhoff 1935: class K of algebras of given signature is variety (defined by equations) iff K closed under H (homomorphic images), S (subalgebras), P (products). Backbone of universal-algebraic structure-theory.
Birkhoff 1935: class K of algebras of given signature is variety (defined by equations) iff K closed under H (homomorphic images), S (subalgebras), P (products). Backbone of universal-algebraic structure-theory.