It is known that one can extract Craig interpolants from so-called local derivations. An interpolant extracted from such a derivation is a boolean combination of formulas occurring in the derivation. However, standard complete proof systems, such as superposition, for theories having the interpolation property are not necessarily complete for local proofs.

In this paper we investigate interpolant extraction from non-local proofs in the superposition calculus and prove a number of general results about interpolant extraction and complexity of extracted interpolants. In particular, we prove that the number of quantifier alternations in first-order interpolants of formulas without quantifier alternations is unbounded. This result has far-reaching consequences for using local proofs as a foundation for interpolating proof systems - any such proof system should deal with formulas of arbitrary quantifier complexity.

To search for alternatives for interpolating proof systems, we consider several variations on interpolation and local proofs. Namely, we give an algorithm for building interpolants from resolution refutations in logic without equality and discuss additional constraints when this approach can be also used for logic with equality. We finally propose a new direction related to interpolation via local proofs in first-order theories.

The authors thank Matthias Baaz who has kindly agreed to deliver this talk.