Abstract. This paper discovers a new phenomenon about the Duffin-Schaeffer conjecture, which claims that λ(∩ ∞ m=1 ∪ ∞ n=m E n ) = 1 if and only if n λ(E n ) = ∞, where λ denotes the Lebesgue measure on R/Z,ψ is any non-negative arithmetical function. Instead of studying ∩ ∞ m=1 ∪∞ n=m E n we introduce an even fundamental object ∪ ∞ n=1 E n and conjecture there exists a universal constant C > 0 such thatIt is shown that this conjecture is equivalent to the Duffin-Schaeffer conjecture. Similar phenomena are found in the fields of p-adic numbers and formal Laurent series. As a byproduct, we answer conditionally a question of Haynes by showing that one can always use the quasi-independence on average method to deduce λ(∩ ∞ m=1 ∪ ∞ n=m E n ) = 1 as long as the Duffin-Schaeffer conjecture is true. We also show among several others that two conjectures of Haynes, Pollington and Velani are equivalent to the Duffin-Schaeffer conjecture, and introduce for the first time a weighted version of the second Borel-Cantelli lemma to the study of the Duffin-Schaeffer conjecture.
Introduction to the Duffin-Schaeffer conjectureThroughout the paper we use the following notations:• p denotes a prime number, • n, h denote positive integers, • ϕ(n) denotes the Euler phi function, • λ denotes the Lebesgue measure on R/Z, r) (B(x, r)) denotes the open (closed) ball with center x and radius r in a given metric space.2000 Mathematics Subject Classification. 11J83.