# Hardness of Bichromatic Closest Pair with Jaccard Similarity

Consider collections A and B of red and blue sets, respectively. Bichromatic Closest Pair is the problem of finding a pair from A×B that has similarity higher than a given threshold according to some similarity measure. Our focus here is the classic Jaccard similarity |a∩b|/|a∪b| for (a,b)∈A×B. We consider the approximate version of the problem where we are given thresholds j_1>j_2 and wish to return a pair from A×B that has Jaccard similarity higher than j_2 if there exists a pair in A×B with Jaccard similarity at least j_1. The classic locality sensitive hashing (LSH) algorithm of Indyk and Motwani (STOC '98), instantiated with the MinHash LSH function of Broder et al., solves this problem in Õ(n^2-δ) time if j_1> j_2^1-δ. In particular, for δ=Ω(1), the approximation ratio j_1/j_2=1/j_2^δ increases polynomially in 1/j_2. In this paper we give a corresponding hardness result. Assuming the Orthogonal Vectors Conjecture (OVC), we show that there cannot be a general solution that solves the Bichromatic Closest Pair problem in O(n^2-Ω(1)) time for j_1/j_2=1/j_2^o(1). Specifically, assuming OVC, we prove that for any δ>0 there exists an ε>0 such that Bichromatic Closest Pair with Jaccard similarity requires time Ω(n^2-δ) for any choice of thresholds j_2<j_1<1-δ, that satisfy j_1< j_2^1-ε.

READ FULL TEXT