Small gaps between products of two primes

Abstract

Let $q_n$ denote the $n^{th}$ number that is a product of exactly two distinct primes. We prove that $$\liminf_{n\to \infty} (q_{n+1}-q_n) \le 6.$$ This sharpens an earlier result of the authors (arXivMath NT/0506067), which had 26 in place of 6. More generally, we prove that if $ν$ is any positive integer, then $$ \liminf_{n\to \infty} (q_{n+ν}-q_n) \le C(ν) = νe^{ν-γ} (1+o(1)).$$ We also prove several other results on the representation of numbers with exactly two prime factors by linear forms.