1. On the structure of abstract algebras;Birkhoff;Proceedings of the Cambridge Philosophical Society,1935
2. Subdirect unions in universal algebras;Birkhoff;Bulletin of the American Mathematical Society,1944
3. A characterization of equational classes of algebras with arbitrary operations;Chang;Bulletin of the American Mathematical Society,1954
4. Two general theorems on direct products of algebras;Chang;Bulletin of the American Mathematical Society,1954
5. A characterization of equational classes of algebras with finitary operations;Chang;Bulletin of the American Mathematical Society,1954