About the proof-theoretic ordinals of weak fixed point theories
Gerhard Jäger, Barbara Primo · Journal of Symbolic Logic · 1992
Abstract This paper presents several proof-theoretic results concerning weak fixed point theories over second order number theory with arithmetic comprehension and full or restricted induction on the natural numbers. It is also shown that there are natural second order theories which are proof-theoretically equivalent but have different proof-theoretic ordinals.