Gerald E. Sacks. Metarecursively enumerable sets and admissible ordinals. Bulletin of the American Mathematical Society, vol. 72 (1966), pp. 59–64. - Gerald E. Sacks. Post's problem, admissible ordinals, and regularity. Transactions of the American Mathematical Society, vol. 124 (1966), pp. 1–23. - Gerald E. Sacks. Metarecursion theory. Sets, models and recursion theory, Proceedings of the Summer School in Mathematical Logic and Tenth Logic Colloquium, Leicester, August-September 1965, edited by John N. Crossley, Studies in logic and the foundations of mathematics, North-Holland Publishing Company, Amsterdam, and Humanities Press, New York, 1967, pp. 243–263. - Graham C. DriscollJr., Metarecursively enumerable sets and their metadegrees. The Journal of symbolic logic, vol. 33 (1968), pp. 389–11.