In number theory, Brun's theorem states that the sum of the reciprocals of the twin primes (pairs of prime numbers which differ by 2) converges to a finite value known as Brun's constant, usually denoted by B2 (sequence A065421 in OEIS). Brun's theorem was proven by Viggo Brun in 1919, and it has historical importance in the introduction of sieve methods.
Asymptotic bounds on twin primes 
The convergence of the sum of reciprocals of twin primes follows from bounds on the density of the sequence of twin primes. Let denote the number of primes p ≤ x for which p + 2 is also prime (i.e. is the number of twin primes with the smaller at most x). Then, for x ≥ 3, we have
That is, twin primes are less frequent than prime numbers by nearly a logarithmic factor. It follows from this bound that the sum of the reciprocals of the twin primes converges, or stated in other words, the twin primes form a small set. In explicit terms the sum
either has finitely many terms or has infinitely many terms but is convergent: its value is known as Brun's constant.
The fact that the sum of the reciprocals of the prime numbers diverges implies that there are infinitely many prime numbers. Because the sum of the reciprocals of the twin primes instead converges, it is not possible to conclude from this result that there are finitely many or infinitely many twin primes. Brun's constant could be an irrational number only if there are infinitely many twin primes.
Numerical estimates 
By calculating the twin primes up to 1014 (and discovering the Pentium FDIV bug along the way), Thomas R. Nicely heuristically estimated Brun's constant to be 1.902160578. Nicely has extended his computation to 1.6×1015 as of 18 January 2010 but this is not the largest computation of its type.
- B2 ≈ 1.902160583104.
There is also a Brun's constant for prime quadruplets. A prime quadruplet is a pair of two twin prime pairs, separated by a distance of 4 (the smallest possible distance). The first prime quadruplets are (5, 7, 11, 13), (11, 13, 17, 19), (101, 103, 107, 109). Brun's constant for prime quadruplets, denoted by B4, is the sum of the reciprocals of all prime quadruplets:
- B4 = 0.87058 83800 ± 0.00000 00005, the error range having a 99% confidence level according to Nicely.
This constant should not be confused with the Brun's constant for cousin primes, prime pairs of the form (p, p + 4), which is also written as B4. Wolf derived an estimate for the Brun-type sums Bn of 4/n.
Further results 
for every and all sufficiently large x.
Many special cases of the above have been proved. Most recently, Jie Wu proved that for sufficiently large x,
where 4.5 corresponds to in the above.
See also 
- Nicely, Thomas R. (18 January 2010). "Enumeration to 1.6*10^15 of the twin primes and Brun's constant". Some Results of Computational Research in Prime Numbers (Computational Number Theory). Retrieved 16 February 2010.
- Damouni, Nadia (1 July 2011). "Dealtalk: Google bid "pi" for Nortel patents and lost". Reuters. Retrieved 6 July 2011.
- Nicely, Thomas R. (26 August 2008). "Enumeration to 1.6×1015 of the prime quadruplets". Some Results of Computational Research in Prime Numbers (Computational Number Theory). Retrieved 9 March 2009.
- Brun, Viggo (1915). "Über das Goldbachsche Gesetz und die Anzahl der Primzahlpaare". Archiv for Mathematik og Naturvidenskab B34 (8).
- Brun, Viggo (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 (in French) 43: 100–104, 124–128.
- Cojocaru, Alina Carmen; Murty, M. Ram (2005). An introduction to sieve methods and their applications. London Mathematical Society Student Texts 66. Cambridge University Press. pp. 73–74. ISBN 0-521-61275-6.
- Landau, E. (1927). Elementare Zahlentheorie. Leipzig, Germany: Hirzel. Reprinted Providence, RI: Amer. Math. Soc., 1990.
- LeVeque, William Judson (1996). Fundamentals of Number Theory. New York: Dover. pp. 1–288. ISBN 0-486-68906-9. Contains a more modern proof.
- Wu, J. (2004). "Chen's double sieve, Goldbach's conjecture and the twin prime problem". Acta Arithmetica 114 (3): pp. 215–273. arXiv:0709.3764. doi:10.4064/aa114-3-2.