1. Which set existence axioms are needed to prove the separable Hahn-Banach theorem?;Brown;Annals Pure Applied Logic,1986
2. Some systems of second order arithmetic and their uses;Friedman;Proc. Internat. Congress of Mathematicians,1975
3. Countable algebra and set existence axioms.;Friedman;Annals Pure Applied Logic,1983
4. Which set existence axioms are needed to prove the Cauchy/Peano theorem for ordinary differential equations?;Simpson;J. Symbolic Logic,1984
5. Non-provability of certain combinatorial properties of finite trees, and Friedman's research on subsystems of second order arithmetic;Simpson,1985