Fragments of Kripke-Platek set theory and the metamathematics of $$\alpha $$ -recursion theory.

The foundation scheme in set theory asserts that every nonempty class has an $$\in $$ -minimal element. In this paper, we investigate the logical strength of the foundation principle in basic set theory and $$\alpha $$ -recursion theory. We take KP set theory without foundation (called KP $$^-$$ ) as the base theory. We show that KP $$^-$$ + $$\Pi _1$$ -Foundation + $$V=L$$ is enough to carry out finite injury arguments in $$\alpha $$ -recursion theory, proving both the Friedberg-Muchnik theorem and the Sacks splitting theorem in this theory. In addition, we compare the strengths of some fragments of KP. [ABSTRACT FROM AUTHOR]