Sign-Rank of k-Hamming Distance is Constant

We prove that the sign-rank of the k Hamming Distance matrix on n bits is $2^{O(k)}$, independent of the number of bits n. This strongly refutes the conjecture of Hatami, Hatami, Pires, Tao, and Zhao (random 2022), and Hatami, Hosseini, and Meng (STOC 2023), repeated in several other papers, that the sign-rank should depend on n. This conjecture would have qualitatively separated margin from sign-rank (or, equivalently, bounded-error from unbounded-error randomized communication). In fact, our technique gives constant sign-rank upper bounds for all matrices which reduce to k-Hamming Distance, as well as large-margin matrices recently shown to be irreducible to k-Hamming Distance.