Variants of the Selberg sieve, and bounded intervals containing many primes

For any $m\ge 1$, let $H_m$ denote the quantity $\liminf_{n \to \infty} (p_{n+m}-p_n)$\mathrm{.}AcelebratedrecentresultofZhangshowedthefinitenessof$H_1$,withtheexplicitbound$H_1 \leq 70000000$. This was then improved by us (the Polymath8 project) to $H_1\le 4680$, and then by Maynard to $H_1 \leq 600$,whoalsoestablishedforthefirsttimeafinitenessresultfor$H_m$for$m \geq 2$,andspecificallythat$H_m \ll m^3 e^{4m}$. If one also assumes the Elliott-Halberstam conjecture, Maynard obtained the bound $H_1\le 12$, improving upon the previous bound $H_1\le 16$ of Goldston, Pintz, and Y{\i}ld{\i}r{\i}m, as well as the bound $H_m\ll m^3{e}^{2m}$. In this paper, we extend the methods of Maynard by generalizing the Selberg sieve further, and by performing more extensive numerical calculations. As a consequence, we can obtain the bound $H_1\le 246$ unconditionally, and $H_1 \leq 6$ under the assumption of the generalized Elliott-Halberstam conjecture. Indeed, under the latter conjecture we show the stronger statement that for any admissible triple $(h_1,h_2,h_3)$, there are infinitely many $n$ for which at least two of $n+h_1,n+h_2,n+h_3$ are prime. We modify the “parity problem” argument of Selberg to show that this result is the best possible that one can obtain from purely sieve-theoretic considerations. For larger $m$, we use the distributional results obtained previously by our project to obtain the unconditional asymptotic bound $H_m\ll m{e}^{(4-\frac{24}{181})m}$, or $H_m \ll m e^{2m}$ under the assumption of the Elliott-Halberstam conjecture. We also obtain explicit upper bounds for $H_m$ when $m=2,3,4,5$.