sentences:  According to the Green–Tao theorem, there exist arbitrarily long sequences of primes in arithmetic progression.Sometimes the phrase may also be used about primes which belong to an arithmetic progression which also contains composite numbers.For example, it can be used about primes in an arithmetic progression of the form formula_3, where "a" and "b" are coprime which according to Dirichlet's theorem on arithmetic progressions contains infinitely many primes, along with infinitely many composites.