1. Lyapunov, A.A., On logical schemes of programs, in Problemy kibernetiki (Problems of Cybernetics), Moscow: Fizmatgiz, 1958, vol. 1, pp. 46–74.
2. Yanov, Yu. I., On logical schemes of algorithms, in Problemy kibernetiki (Problems of Cybernetics), Moscow: Fizmatgiz, 1958, vol. 1, pp. 75–127.
3. Podlovchenko, R.I., Hierarchy of program models, Programmirovanie, 1981, no. 2, pp. 3–14.
4. Podlovchenko, R.I., Recursive programs and hierarchies of their models, Programmirovanie, 1991, no. 6, pp. 44–51.
5. Grzegorczyk, A., Rozprawy Matematyczne, vol. 4: Some Classes of Recursive Functions, Warsaw: Math. Inst., Polish Acad. Sci., 1953.