Presburger Arithmetic with algebraic scalar multiplications

Abstract

We consider Presburger arithmetic (PA) extended by scalar multiplication by an algebraic irrational number $α$, and call this extension $α$-Presburger arithmetic ($α$-PA). We show that the complexity of deciding sentences in $α$-PA is substantially harder than in PA. Indeed, when $α$ is quadratic and $r\geq 4$, deciding $α$-PA sentences with $r$ alternating quantifier blocks and at most $c\ r$ variables and inequalities requires space at least $K 2^{\cdot^{\cdot^{\cdot^{2^{C\ell(S)}}}}}$ (tower of height $r-3$), where the constants $c, K, C>0$ only depend on $α$, and $\ell(S)$ is the length of the given $α$-PA sentence $S$. Furthermore deciding $\exists^{6}\forall^{4}\exists^{11}$ $α$-PA sentences with at most $k$ inequalities is PSPACE-hard, where $k$ is another constant depending only on~$α$. When $α$ is non-quadratic, already four alternating quantifier blocks suffice for undecidability of $α$-PA sentences.