Twin primes are pairs of primes of the form (, ). The term "twin prime" was coined by Paul Stäckel (18621919; Tietze 1965, p. 19). The first few twin primes are for , 6, 12, 18, 30, 42, 60, 72, 102, 108, 138, 150, 180, 192, 198, 228, 240, 270, 282, ... (OEIS A014574). Explicitly, these are (3, 5), (5, 7), (11, 13), (17, 19), (29, 31), (41, 43), ... (OEIS A001359 and A006512).
All twin primes except (3, 5) are of the form .
It is conjectured that there are an infinite number of twin primes (this is one form of the twin prime conjecture), but proving this remains one of the most elusive open problems in number theory. An important result for twin primes is Brun's theorem, which states that the number obtained by adding the reciprocals of the odd twin primes,
(1)

converges to a definite number ("Brun's constant"), which expresses the scarcity of twin primes, even if there are infinitely many of them (Ribenboim 1996, p. 201). By contrast, the series of all prime reciprocals diverges to infinity, as follows from the Mertens second theorem by letting .
The following table gives the first few for the twin primes (, ), cousin primes (, ), sexy primes (, ), etc.
pair  OEIS  first member 
(, )  A001359  3, 5, 11, 17, 29, 41, 59, 71, ... 
(, )  A023200  3, 7, 13, 19, 37, 43, 67, 79, ... 
(, )  A023201  5, 7, 11, 13, 17, 23, 31, 37, ... 
(, )  A023202  3, 5, 11, 23, 29, 53, 59, 71, ... 
(, )  A023203  3, 7, 13, 19, 31, 37, 43, 61, ... 
(, )  A046133  5, 7, 11, 17, 19, 29, 31, 41, ... 
Let be the number of twin primes and such that . It is not known if there are an infinite number of such primes (Wells 1986, p. 41; Shanks 1993), but it seems almost certain to be true (Hardy and Wright 1979, p. 5).
J. R. Chen has shown there exists an infinite number of primes such that has at most two factors (Le Lionnais 1979, p. 49). Brun proved that there exists a computable integer such that if , then
(2)

(Ribenboim 1996, p. 261). It has been shown that
(3)

written more concisely as
(4)

where is known as the twin primes constant and is another constant. The constant has been reduced to (Fouvry and Iwaniec 1983), (Fouvry 1984), 7 (Bombieri et al. 1986), 6.9075 (Fouvry and Grupp 1986), 6.8354 (Wu 1990), and 6.8325 (Haugland 1999). The latter calculation involved evaluation of 7fold integrals and fitting of three different parameters.
Hardy and Littlewood (1923) conjectured that (Ribenboim 1996, p. 262), and that is asymptotically equal to
(5)

This result is sometimes called the strong twin prime conjecture and is a special case of the ktuple conjecture. A necessary (but not sufficient) condition for the twin prime conjecture to hold is that the prime gaps constant, defined by
(6)

where is the th prime and is the prime difference function, satisfies .
Wolf notes that the formula
(7)

(which has asymptotic growth ) agrees with numerical data much better than does , although not as well as .
Extending the search done by Brent in 1974 or 1975, Wolf has searched for the analog of the Skewes number for twins, i.e., an such that changes sign. Wolf checked numbers up to and found more than sign changes. From this data, Wolf conjectured that the number of sign changes for of is given by
(8)

Proof of this conjecture would also imply the existence an infinite number of twin primes.
The largest known twin primes as of Sep. 2016 correspond to
(9)

each having decimal digits and found by PrimeGrid on Dec. 25, 2011 (http://primes.utm.edu/top20/page.php?id=1#records).
In 1995, Nicely discovered a flaw in the Intel® PentiumTM microprocessor by computing the reciprocals of and , which should have been accurate to 19 decimal places but were incorrect from the tenth decimal place on (Cipra 1995, 1996; Nicely 1996).
If , the integers and form a pair of twin primes iff
(10)

where is a pair of twin primes iff
(11)

(Ribenboim 1996, p. 259). S. M. Ruiz has found the unexpected result that are twin primes iff
(12)

for , where is the floor function.
The values of were found by Brent (1976) up to . T. Nicely calculated them up to in his calculation of Brun's constant. Fry et al. (2001) and Sebah (2002) independently obtained using distributed computation. The following table gives known values of (OEIS A007508; Ribenboim 1996, p. 263; Nicely 1999; Sebah 2002).
35  
205  
1224  
It is conjectured that every even number is a sum of a pair of twin primes except a finite number of exceptions whose first few terms are 2, 4, 94, 96, 98, 400, 402, 404, 514, 516, 518, ... (OEIS A007534; Wells 1986, p. 132).