Let 𝑓 : {0, 1} 𝑛 → {0, 1} be a boolean function, and let 𝑓 ∧ (𝑥, 𝑦) = 𝑓 (𝑥 ∧𝑦) denote the AND-function of 𝑓 , where 𝑥 ∧𝑦 denotes bit-wise AND. We study the deterministic communication complexity of 𝑓 ∧ and show that, up to a log 𝑛 factor, it is bounded by a polynomial in the logarithm of the real rank of the communication matrix of 𝑓 ∧ . This comes within a log 𝑛 factor of establishing the log-rank conjecture for AND-functions with no assumptions on 𝑓 . Our result stands in contrast with previous results on special cases of the logrank conjecture, which needed significant restrictions on 𝑓 such as monotonicity or low F 2 -degree. Our techniques can also be used to prove (within a log 𝑛 factor) a lifting theorem for AND-functions, stating that the deterministic communication complexity of 𝑓 ∧ is polynomially related to the AND-decision tree complexity of 𝑓 .The results rely on a new structural result regarding boolean functions 𝑓 : {0, 1} 𝑛 → {0, 1} with a sparse polynomial representation, which may be of independent interest. We show that if the polynomial computing 𝑓 has few monomials then the set system of the monomials has a small hitting set, of size poly-logarithmic in its sparsity. We also establish extensions of this result to multi-linear polynomials 𝑓 : {0, 1} 𝑛 → R with a larger range.
CCS CONCEPTS• Theory of computation → Communication complexity; Oracles and decision trees.