Dear SeqFans! Let a(0) = p and a(n) = 2 a(n-1) + 1. Note that a(n) = (p+1) 2^n - 1. Are there primes p such that a(n) is composite for every n > 0 ? Best regards, Thomas Ordowski _______________________ https://en.wikipedia.org/wiki/Riesel_number