1. On the computational complexity of algorithms;Hartmanis, J.;Trans. Amer. Math. Soc.,1965
2. M. O. Rabin, Degree of difficulty of computing a function, and a partial ordering of recursive sets, Hebrew University, Jerusalem, 1960.
3. Classes of predictably computable functions;Ritchie, Robert W.;Trans. Amer. Math. Soc.,1963