Dirichlet’s theorem

Dirichlet’s theorem, statement that there are infinitely many prime numbers contained in the collection of all numbers of the form na + b, in which the constants a and b are integers that have no common divisors except the number 1 (in which case the pair are known as being relatively prime) and the variable n is any natural number (1, 2, 3, …). For instance, because 3 and 4 are relatively prime, there must be infinitely many primes among numbers of the form 4n + 3 (e.g., 7 when n = 1, 11 when n = 2, 19 when n = 4, and so forth). Conjectured by the late 18th–early 19th-century German mathematician Carl Friedrich Gauss, the statement was first proved in 1826 by the German mathematician Peter Gustav Lejeune Dirichlet.