Counting on Nowhere Dense Classes

Abstract: For every effectively nowhere dense class $\mathcal{C}$ of relational structures, we present an algorithm that runs an almost-linear-time preprocessing step on a given structure $\mathcal{A} \in \mathcal{C}$ and a first-order formula $φ(x_1, \dots, x_k, y_1, \dots, y_\ell)$. After the preprocessing, whenever given a tuple $\bar{v} \in A<sup>k$, the algorithm computes the number of tuples $\bar{w} \in A<sup>\ell$ that satisfy $\mathcal{A} \models φ(\bar{v}, \bar{w})$ in constant time. Building on this, we provide an algorithm for constant-time query answering and constant-delay enumeration after almost-linear-time preprocessing for the recently introduced logic clique-guarded first-order logic with counting (cgFOC) on effectively nowhere dense classes. This generalises the testing and enumeration results for first-order logic [Schweikardt, Segoufin, and Vigny, JACM 2022] and the evaluation result for the first-order logic with counting FOC1 [Grohe and Schweikardt, PODS 2018] on nowhere dense classes.