,
Santiago Guzmán-Pro
Creative Commons Attribution 4.0 International license
Extensional ESO is a fragment of existential second-order logic (ESO) that captures the following family of problems. Given a fixed ESO sentence Ψ and an input structure A the task is to decide whether there is an extension B of A that satisfies the first-order part of Ψ, i.e., a structure B such that R^A ⊆ R^B for every existentially quantified predicate R of Ψ, and R^A = R^B for every non-quantified predicate R of Ψ. In particular, extensional ESO describes all pre-coloured finite-domain constraint satisfaction problems (CSPs). In this paper we study the computational power of extensional ESO; we ask, for which problems in NP is there a polynomial-time equivalent problem in extensional ESO? One of our main results states that extensional ESO has the same computational power as hereditary first-order logic. We also characterize the computational power of the fragment of extensional ESO with monotone universal first-order part in terms of finitely bounded CSPs. These results suggest a rich computational power of this logic, and we conjecture that extensional ESO captures NP-intermediate problems. We further support this conjecture by showing that extensional ESO can express current candidate NP-intermediate problems such as Graph Isomorphism, and Monotone Dualization (up to polynomial-time equivalence). On the other hand, another main result proves that extensional ESO does not have the full computational power of NP: there are problems in NP that are not polynomial-time equivalent to a problem in extensional ESP (unless E=NE).
@InProceedings{bodirsky_et_al:LIPIcs.LICS.2026.20,
author = {Bodirsky, Manuel and Guzm\'{a}n-Pro, Santiago},
title = {{On the Computational Power of Extensional ESO}},
booktitle = {41st Annual Symposium on Logic in Computer Science (LICS 2026)},
pages = {20:1--20:26},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-434-5},
ISSN = {1868-8969},
year = {2026},
volume = {380},
editor = {Faggian, Claudia and Katoen, Joost-Pieter},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.LICS.2026.20},
URN = {urn:nbn:de:0030-drops-268073},
doi = {10.4230/LIPIcs.LICS.2026.20},
annote = {Keywords: Existential second-order logic, constraint satisfaction problem, complexity classification, Hereditary first-order logic, NP-intermediate problems}
}