The adjacent fragment and Quine’s limits of decision

Abstract

We introduce the adjacent fragment AF of first-order logic, obtained by restricting the sequences of variables occurring as arguments in atomic formulas. The adjacent fragment generalizes (after a routine renaming) the two-variable fragment of first-order logic as well as the so-called fluted fragment. We show that the adjacent fragment has the finite model property, and that the satisfiability problem for its k-variable sub-fragment is in (k−1)-NEXPTIME. Using known results on the fluted fragment, it follows that the satisfiability problem for the whole adjacent fragment is TOWER-complete. We additionally consider the effect of the adjacency requirement on the well-known guarded fragment of first-order logic, whose satisfiability problem is 2EXPTIME-complete. We show that the satisfiability problem for the intersection of the adjacent and guarded adjacent fragments remains 2EXPTIME-hard. Finally, we show that any relaxation of the adjacency condition on the allowed order of variables in argument sequences yields a logic whose satisfiability and finite satisfiability problems are undecidable.