A Density Version of the Vinogradov Three Primes Theorem

We prove that if A is a subset of the primes, and the lower density of A in the primes is larger than 5/8, then all sufficiently large odd positive integers can be written as the sum of three primes in A. The constant 5/8 in this statement is the best possible.Comment: 17 pages. To appear in Duke Math Journal. This is the final version, incorporating referee's suggestion