In the field of number theory, the Brun sieve (also called Brun's pure sieve) is a technique for estimating the size of "sifted sets" of positive integers which satisfy a set of conditions which are expressed by congruences. It was developed by Viggo Brun in 1915.
Let A be a set of positive integers ≤ x and let P be a set of primes. For each p in P, let Ap denote the set of elements of A divisible by p and extend this to let Ad the intersection of the Ap for p dividing d, when d is a product of distinct primes from P. Further let A1 denote A itself. Let z be a positive real number and P(z) denote the primes in P ≤ z. The object of the sieve is to estimate
We assume that |Ad| may be estimated by
where w is a multiplicative function and X = |A|. Let
Brun's pure sieve 
This formulation is from Cojocaru & Murty, Theorem 6.1.2. With the notation as above, assume that
- |Rd| ≤ w(d) for any squarefree d composed of primes in P ;
- w(p) < C for all p in P ;
where C, D, E are constants.
In particular, if log z < c log x / log log x for a suitably small c, then
- Brun's theorem: the sum of the reciprocals of the twin primes converges;
- Schnirelmann's theorem: every even number is a sum of at most C primes (where C can be taken to be 6);
- There are infinitely many pairs of integers differing by 2, where each of the member of the pair is the product of at most 9 primes;
- Every even number is the sum of two numbers each of which is the product of at most 9 primes.
The last two results were superseded by Chen's theorem.
- Viggo Brun (1915). "Über das Goldbachsche Gesetz und die Anzahl der Primzahlpaare". Archiv for Mathematik og Naturvidenskab B34 (8).
- Viggo Brun (1919). "La série 1/5+1/7+1/11+1/13+1/17+1/19+1/29+1/31+1/41+1/43+1/59+1/61+..., où les dénominateurs sont nombres premiers jumeaux est convergente ou finie". Bulletin des Sciences Mathématiques 43: 100–104,124–128.
- Alina Carmen Cojocaru; M. Ram Murty (2005). An introduction to sieve methods and their applications. London Mathematical Society Student Texts 66. Cambridge University Press. pp. 80–112. ISBN 0-521-61275-6.
- George Greaves (2001). Sieves in number theory. Ergebnisse der Mathematik und ihrer Grenzgebiete (3. Folge) 43. Springer-Verlag. pp. 71–101. ISBN 3-540-41647-1.
- Heini Halberstam; H.E. Richert (1974). Sieve Methods. Academic Press. ISBN 0-12-318250-6.
- Christopher Hooley (1976). Applications of sieve methods to the theory of numbers. Cambridge University Press. ISBN 0-521-20915-3..