math.NT — Number Theory
A lower explicit threshold for Chen's theorem under GRH
Assume the Generalized Riemann Hypothesis (GRH). We show that every even integer can be written as , where is a prime and is either a prime or a product of two primes. The result is conditional on GRH and on Theorems 5, 6, 7 and Lemma 9 of Bordignon and Starichkova, Ramanujan J. 64 (2024), together with the auxiliary lemmas, Table 1 and display (6) that their Section 5.4 uses, at new parameter values; they obtain . It is not a new method: it is a re-choice of the parameters and in their argument, combined with the observation that Chen's theorem needs only that the Section 5.4 main-term combination be positive, not that it exceed , because the inequality from the proof of their Corollary 1 gives as soon as . The arithmetic of Section 5.4 at the new parameters and the parameter range inequalities are verified unconditionally in Lean 4.