Connecting real and hyperarithmetical analysis

Abstract

Going back to Kreisel in the sixties, hyperarithmetical analysis is a cluster of logical systems just beyond arithmetical comprehension. Only recently natural examples of theorems from the mathematical mainstream were identified that fit this category. In this paper, we provide many examples of theorems of real analysis that sit within the range of hyperarithmetical analysis, namely between the higher-order version of ${\Sigma }_1^1$-${\mathsf{AC}}_0$ and $\mathsf{weak}$-${\Sigma }_1^1$-${\mathsf{AC}}_0$, working in Kohlenbach’s higher-order framework. Our example theorems are based on the Jordan decomposition theorem, unordered sums, metric spaces, and semi-continuous functions. Along the way, we identify a couple of new systems of hyperarithmetical analysis.