We prove here that the intuitionistic theory T0↾+UMIDN, or even EETJ↾+UMIDN, of Explicit
Mathematics has the
strength of Π21-CA0. In Section 1 we give a double-negation translation for the
classical second-order μ-calculus, which was shown in [Moe02] to have the
strength of Π21-CA0. In Section 2 we interpret the intuitionistic μ-calculus
in the theory EETJ↾+UMIDN.
The question about the strength of monotone inductive definitions in T0
was asked by S. Feferman in 1982, and — assuming
classical logic — was addressed by M. Rathjen.