# A proof of conservativity of fixed points over Heyting arithmetic via truth

## Mattias Granberg Olsson, FLoV

I will present work in progress (together with Graham Leigh) on a novel proof of the conservativity of the intuitionistic fix-point theory over Heyting arithmetic (HA), originally proved in full generality by Arai [1].

We make use of the work of van den Berg and van Slooten [2] on realizability in Heyting arithmetic over Beeson’s logic of partial terms (HAP). Let IF be the extension of Heyting arithmetic by fix-points, denoted \hat{ID}^{i}_{1} in the literature. The proof is divided into four parts: First we extend the inclusion of HA into HAP to IF into a similar theory IFP in the logic of partial terms. We then show that every theorem of this theory provably has a realizer in the theory IFP(Λ) of fix-points for almost negative operator forms only. Constructing a hierarchy stratifying the class of almost negative formulae and partial truth predicates for this hierarchy, we use Gödel’s diagonal lemma to show IFP(Λ) is interpretable in HAP. Finally we use use the result of [2] that adding the schema of “self-realizability” for arithmetic formulae to HAP is conservative over HA. The result generalises the work presented at my half-time seminar 2020-08-28.

# References

[1] Toshiyasu Arai. Quick cut-elimination for strictly positive cuts. Annals of Pure and Applied Logic, 162(10):807–815, 2011.

[2] Benno van den Berg and Lotte van Slooten. Arithmetical conservation results. Ind- agationes Mathematicae, 29:260–275, 2018.