An Exponential Succinctness Gap between Three-Variable Logic and the Calculus of Relations

Abstract: Three-variable first-order logic (FO3) and the calculus of relations (CoR) define the same binary queries, an equivalence going back to Tarski in the 1940s. While the classical translation $\text{FO3} \Rightarrow \text{CoR}$ is exponential, we prove that this blow-up is unavoidable, resolving a long-standing open question. We construct positive formulas $\varphi$ with a single quantifier whose equivalent terms require size $2<sup>{Ω(|\varphi|)}$, even over finite structures and circuit representations with subterm sharing. Our proof uses a preservation argument over a single finite structure. This approach applies beyond our primary question, establishing the lower bound even for size-specific circuits and bounded-error randomized circuits, and yielding an analogous exponential gap for the matrix query language MATLANG.