This paper presents a novel proof of the conservativity of the intuitionistic theory of strictly positive fixpoints, $$\widehat{{\textrm{ID}}}{}_{1}^{{\textrm{i}}}{}$$ ID ^ 1 i, over Heyting arithmetic ($${\textrm{HA}}$$ HA ), originally proved in full generality by Arai (Ann Pure Appl Log 162:807–815, 2011. https://doi.org/10.1016/j.apal.2011.03.002). The proof embeds $$\widehat{{\textrm{ID}}}{}_{1}^{{\textrm{i}}}{}$$ ID ^ 1 i into the corresponding theory over Beeson’s logic of partial terms and then uses two consecutive interpretations, a realizability interpretation of this theory into the subtheory generated by almost negative fixpoints, and (...) a direct interpretation into Heyting arithmetic with partial terms using a hierarchy of satisfaction predicates for almost negative formulae. It concludes by applying van den Berg and van Slooten’s result (Indag Math 29:260–275, 2018. https://doi.org/10.1016/j.indag.2017.07.009) that Heyting arithmetic with partial terms plus the schema of self realizability for arithmetic formulae is conservative over $${\textrm{HA}}$$ HA. (shrink)

We show that the existence of a weakly compact cardinal over the Zermelo–Fraenkel's set theory ZF is proof-theoretically reducible to iterations of Mostowski collapsings and Mahlo operations.