1. Smullyan, R. M.: Theory of formal systems. Ann. of Math. Studies47, Princeton University Press, Princeton, N.J., 1961.
2. Trahtenbrot, B.A.: On recursive inseparability, Doklady Akademii Nauk SSSR88, 953–956 (1953).
3. Wang, H.: Dominoes and the AEA case of the decision problem. In: Proceedings of the symposium on Math. Theory of Automata; Politechnic Institute of Brooklyn, New York 1962.