1. Algebraic theories;Adámek,2011
2. On the structure of abstract algebras;Birkhoff;Math. Proc. Camb. Philos. Soc.,1935
3. Equivalence of consequence operations;Blok;Stud. Log.,2006
4. Philosophy and Model Theory;Button,2018
5. Introduction to Mathematical Logic, vol. I;Church,1956