PDDL Axioms Are Equivalent to Least Fixed Point Logic (Extended Version)
Abstract
Axioms are a feature of the Planning Domain Definition Language PDDL that can be considered as a generalization of database query languages such as Datalog. The PDDL standard restricts negative occurrences of predicates in axiom bodies to predicates that are directly set by actions and not derived by axioms. In the literature, authors often deviate from this limitation and only require that the set of axioms is stratifiable. We show that both variants can express exactly the same queries as least fixed point logic. They are thus strictly more expressive than stratified Datalog, which aligns with another restriction on axioms occasionally considered in the planning literature. Complementing this theoretical analysis, we also present a compilation that eliminates negative occurrences of derived predicates from PDDL axioms.
Cite
@article{arxiv.2510.14412,
title = {PDDL Axioms Are Equivalent to Least Fixed Point Logic (Extended Version)},
author = {Claudia Grundke and Gabriele Röger},
journal= {arXiv preprint arXiv:2510.14412},
year = {2026}
}
Comments
v1: Extended version of "Eliminating Negative Occurrences of Derived Predicates from PDDL Axioms" at the joint KR/ICAPS 2025 workshop KRPlan; v2: Extended version of "PDDL Axioms Are Equivalent to Least Fixed Point Logic" (ICAPS 2026). It adds the result on the equivalence of PDDL axioms and LFP and does now longer contain the deeper analysis of the blow-up incurred by the compilation