1. Bahareh Afshari, Relative computability and the proof-theoretic strength of some theories, Ph.D. Thesis, University of Leeds, U.K., 2008
2. Elementary descent recursion and proof theory;Friedman;Annals of Pure and Applied Logic,1995
3. Harvey Friedman, Antonio Montalban, Andreas Weiermann, Phi function, draft, 2007
4. Jean-Yves Girard, Proof Theory and Logical Complexity, vol. 1, Bibliopolis, Napoli, 1987
5. Reverse mathematics and ordinal exponentiation;Hirst;Annals of Pure and Applied Logic,1994