1. Yu. L. Ershov, “A Certain Hierarchy of Sets, I,” Algebra Logika 7(1), 47–74 (1968).
2. Yu. L. Ershov, “A Certain Hierarchy of Sets, II,” Algebra Logika 7(4), 15–47 (1968).
3. Yu. L. Ershov, “A Certain Hierarchy of Sets, III,” Algebra Logika 9(1), 34–51 (1970).
4. E.M. Gold, “Limiting Recursion,” J. Symbolic Logic 30(1), 28–48 (1965).
5. H. Putnam, “Trial and Error Predicates and the Solution to a Problem ofMostowski,” J. Symbolic Logic 30(1), 49–57 (1965).