A new model construction by making a detour via intuitionistic theories II: Interpretability lower bound of Feferman's explicit mathematics T0T0

We partially solve a long-standing problem in the proof theory of explicit mathematics or the proof theory in general. Namely, we give a lower bound of Feferman's system T0T0 of explicit mathematics (but only when formulated on classical logic) with a concrete interpretation of the subsystem Σ21-AC+(BI) of second order arithmetic inside T0T0. Whereas a lower bound proof in the sense of proof-theoretic reducibility or of ordinal analysis was already given in 80s, the lower bound in the sense of interpretability we give here is new.We apply the new interpretation method developed by the author and Zumbrunnen [37], which can be seen as the third kind of model construction method for classical theories, after Cohen's forcing and Krivine's classical realizability. It gives us an interpretation between classical theories, by composing interpretations between intuitionistic theories.