Simon Kochen. Topics in the theory of definition. The theory of models, Proceedings of the 1963 International Symposium at Berkeley, edited by J. W. Addison, Leon Henkin, and Alfred Tarski, Studies in logic and the foundations of mathematics, North-Holland Publishing Company, Amsterdam1965, pp. 170–176. - Walter Felscher. On criteria of definability. Proceedings of the American Mathematical Society, vol. 19 (1968), pp. 834–836.