EUF$^n$: A Decidable Extension to the Theory of Equality with Uninterpreted Functions

Abstract

The theory of Equality with Uninterpreted Functions (EUF) is fundamental to constraint solving and program verification. Uninterpreted functions abstract concrete implementations, enabling generalization and simplification of theorems and proofs. However, standard EUF restricts function composition to fixed finite depths (\emph{e.g.}, $f^k(x)$ where $k$ is constant). This work extends EUF to EUF$^n$, supporting \emph{parametric composition depth} for unary functions (\emph{e.g.}, $f^n(x)$ where $n$ is a natural number variable). An EUF$^n$ formula can be viewed as a disjunction of infinitely many EUF formulas, each instantiated by an assignment of natural numbers. Its satisfiability is defined by the satisfiability of at least one such instantiated EUF formula. We establish the decidability of the EUF$^n$ satisfiability problem via a \emph{conditional congruence graph (CCG)} algorithm. This approach generalizes the standard congruence closure procedure by maintaining conditional equivalence relations between terms. The algorithm reduces the satisfiability problem to deciding existential sentences in Presburger arithmetic with divisibility, which is a decidable problem, thereby yielding a decision procedure for the quantifier-free fragment of EUF$^n$ with a 2NEXPTIME complexity upper bound. The enhanced expressiveness of EUF$^n$ enables new applications: (1) Encoding a decidable subclass of interleaved Dyck reachability problems where existing over/under-approximations produce false positives/negatives, and (2) Encoding a new decidable subclass of uninterpreted program verification problems.