Near-optimal Lower Bounds on Quantifier Depth and Weisfeiler-Leman Refinement Steps

We prove near-optimal tradeoffs for quantifier depth (also called quantifier rank) versus number of variables in first-order logic by exhibiting pairs of n-element structures that can be distinguished by a k-variable firstorder sentence but where every such sentence requires quantifier depth at least n(Omega(k/ log k)). Our tradeoffs also apply to first-order counting logic and, by the known connection to the k-dimensional Weisfeiler-Leman algorithm, imply near-optimal lower bounds on the number of refinement iterations. A key component in our proof is the hardness condensation technique introduced by Razborov in the context of proof complexity. We apply this method to reduce the domain size of relational structures while maintaining the minimal quantifier depth needed to distinguish them in finite variable logics.