In number theory, the Elliott–Halberstam conjecture is a conjecture about the distribution of prime numbers in arithmetic progressions. It has many applications in sieve theory. It is named for Peter D. T. A. Elliott and Heini Halberstam.
To state the conjecture requires some notation. Let denote the number of primes less than or equal to x. If q is a positive integer and a is coprime to q, we let , denote the number of primes less than or equal to x which are equal to a modulo q. Dirichlet's theorem on primes in arithmetic progressions then tells us that
where a is coprime to q and is Euler's totient function. If we then define the error function
where the max is taken over all a coprime to q, then the Elliott–Halberstam conjecture is the assertion that for every θ < 1 and A > 0 there exists a constant C > 0 such that
for all x > 2.
This conjecture was proven for all θ < 1/2 by Enrico Bombieri and A. I. Vinogradov (the Bombieri–Vinogradov theorem, sometimes known simply as "Bombieri's theorem"); this result is already quite useful, being an averaged form of the generalized Riemann hypothesis. It is known that the conjecture fails at the endpoint θ = 1.
The Elliott–Halberstam conjecture has several consequences. One striking one is the result announced by Dan Goldston, János Pintz, and Cem Yıldırım, which shows (assuming this conjecture) that there are infinitely many pairs of primes which differ by at most 16. In November 2013, James Maynard showed that subject to the Elliott–Halberstam conjecture, one can show the existence of infinitely many pairs of consecutive primes that differ by at most 12. In August 2014, Polymath group showed that subject to the generalized Elliott–Halberstam conjecture, one can show the existence of infinitely many pairs of consecutive primes that differ by at most 6. 
- Bombieri, E. (1965). "On the large sieve". Mathematika 12: 201–225. doi:10.1112/s0025579300005313.
- Elliott, P. D. T. A.; Halberstam, H. (1968). "A conjecture in prime number theory". Symp. Math. 4: 59–72.
- Vinogradov, A. I. (1965). "The density hypothesis for Dirichlet L-series". Izv. Akad. Nauk SSSR Ser. Mat. (in Russian) 29 (4): 903–934. MR 197414.
- Soundararajan, K. (2007). "Small gaps between prime numbers: The work of Goldston–Pintz–Yıldırım". Bull. AMS 44 (1): 1–18. doi:10.1090/S0273-0979-06-01142-6.