1. Cooper, B.S.: Degrees of Unsolvability. Ph.D thesis, Leicester University, Leicester, England (1971)
2. Downey, R.G.: Some computability-theoretic aspects of reals and randomness. In: The Notre Dame lectures. Assoc. Symbol. Logic. Lect. Notes Log., vol. 18, pp. 97–147. Urbana, IL (2005)
3. Downey, R.G., Hirschfeldt, D.R.: Algorithmic Randomness and Complexity. Springer, Heidelberg, Monograph to be published
4. Ershov, Y.L.: A certain hierarchy of sets. i, ii, iii. (Russian). Algebra i Logika. 7(1), 47–73 (1968), 7(4), 15–47 (1968), 9, 34–51 (1970)
5. Gold, E.M.: Limiting recursion. J. Symbolic Logic 30, 28–48 (1965)