1. Adam, A. 1968. “Truth functions and the problem of their realization by two-terminal graphs”. Budapest: Akadémiai Kiadö.
2. Heterogeneous algebras
3. De Bouvère, K.L. 1959. “A method in proofs of undefinability”. Amsterdam: North-Holland.
4. Burris, S. and Sankappanavar, H.P. 1981. “A course in universal algebra”. Berlin: Springer.
5. Church, A. 1956. “Introduction to mathematical logic”. Vol. 1, Princeton University Press.