Quasipolynomial bounds for the corners theorem

Abstract: Let $G$ be a finite abelian group and $A$ be a subset of $G \times G$ which is corner-free, meaning that there are no $x, y \in G$ and $d \in G \setminus {0}$ such that $(x, y)$, $(x+d, y)$, $(x, y+d) \in A$. We prove that [|A| \le |G|^2 \cdot \exp(-(\log |G|)^{\Omega(1)}).] As a consequence, we obtain polynomial (in the input length) lower bounds on the non-deterministic communication complexity of Exactly-N in the 3-player Number-on-Forehead model. We also obtain the first "reasonable'' lower bounds on the coloring version of the $3$-dimensional corners problem and equivalently the deterministic communication complexity of Exactly-N in the 4-player Number-on-Forehead model.