Abstract

Tanaka proved a powerful generalization of Friedman’s self-embedding theorem that states that given a countable nonstandard model \\) of the subsystem \ of second order arithmetic, and any element m of \, there is a self-embedding j of \\) onto a proper initial segment of itself such that j fixes every predecessor of m. Here we extend Tanaka’s work by establishing the following results for a countable nonstandard model \\ \)of \ and a proper cut \ of \:Theorem A. The following conditions are equivalent: \ is closed under exponentiation. There is a self-embedding j of \\) onto a proper initial segment of itself such that I is the longest initial segment of fixed points of j.Theorem B. The following conditions are equivalent: \ is a strong cut of \ and \ There is a self-embedding j of \\) onto a proper initial segment of itself such that \ is the set of all fixed points of j.