Diophantine approximation
The Duffin–Schaeffer conjecture is a conjecture (now a theorem) in mathematics, specifically, the Diophantine approximation proposed by R. J. Duffin and A. C. Schaeffer in 1941. It states that if formula_1 is a real-valued function taking on positive values, then for almost all formula_2 (with respect to Lebesgue measure), the inequality