1. C. T. Chong and K. J. Mourad, The Friedberg-Muchnik Theorem without Σ1 induction, in preparation
2. C. T. Chong and K. J. Mourad, Post's problem and singularity, in preparation
3. S. D. Friedman, Post's problem without admissibility, Advances in Math. 35 (1980), 30–49
4. S. D. Friedman, β recursion theory, Trans. Amer. Math. Soc. 255 (1979), 173–200
5. S. D. Friedman, Negative solutions to Post's problem I, in: Generalized Recursion Theory II, North-Holland, 1978, 127–134