Two-Variable Logic with Arbitrarily Many Successor Relations

Abstract

We study two-variable first-order logic with a finite, input-dependent number of distinguished successor relations and arbitrary additional unary and binary predicates. Each distinguished relation must be the exact immediate-successor relation of some linear order on the common domain; the orders themselves are not available in the language and may have arbitrary order types. We prove that satisfiability belongs to 2NEXPTIME, uniformly in the number of successors. The proof gives finite certificates for possibly infinite models. A closure operation separates local neighbourhood descriptions, called star types, into those of bounded multiplicity and those that can be realized infinitely often. The bounded part is represented explicitly. Outside it, integer height vectors allow overlapping successor requirements to be matched without creating cycles or unintended identifications. Ordering the resulting path components densely then recovers exact successor relations. Consistent assignments of the additional binary predicates handle witnesses involving the bounded part. The same certificates decide infinite satisfiability in 2NEXPTIME and give a doubly exponential threshold above which a finite model guarantees an infinite model. A doubly exponentially large finite core can be forced with only two successors and unary predicates. For finite models, we prove that satisfiability subject to a bound on the total number of reference adjacencies lost by the other orders is NEXPTIME-complete, even when the budget is encoded in binary and the number of successors is part of the input.