This technical note applies the arithmetic function kappac (n) = prod₃|₍ (d+c) to the Mersenne sequence Mₚ = 2ᵖ - 1. A normalized quantity ac (Mₚ) is introduced, equal to 1 exactly when Mₚ is prime and strictly between 0 and 1 otherwise. This yields the exact formulaMP (P) = floor (sum_{p = 0.
Christophe POKORSKI (Sun,) studied this question.