OFFSET
1,3
COMMENTS
All terms correspond to certified primes using PFGW from primeform group.
EXAMPLE
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.
CROSSREFS
KEYWORD
more,nonn
AUTHOR
Pierre CAMI, Oct 26 2008
EXTENSIONS
Edited by N. J. A. Sloane, Nov 27 2008 at the suggestion of R. J. Mathar.
STATUS
approved