1,3

All terms correspond to certified primes using PFGW from primeform group

Table of n, a(n) for n=1..30.

6*1*(2^2-1)-1 = 17 is prime as is 19 so a(1) = 1. 6*1*(2^3-1)-1 = 41 is prime as is 43 so a(2) = 1. 6*1*(2^5-1)-1 = 185 is not prime and 187 is not prime; 6*2*(2^5-1)-1 = 381 is not prime but 383 is prime so a(3) = 2.

Sequence in context: A127124 A127136 A239101 * A245025 A257495 A120025

Adjacent sequences: A145980 A145981 A145982 * A145984 A145985 A145986

more,nonn

Pierre CAMI, Oct 26 2008

Edited by N. J. A. Sloane, Nov 27 2008 at the suggestion of R. J. Mathar.

approved