1,1

All a(n), except a(2) = 5, are of the form (2^p + 1)/3 - the Wagstaff primes A000979[n] = {3,11,43,683,2731,43691,174763,...}. Indices of prime Jacobsthal numbers are listed in A107036[n] = {3,4,5,7,11,13,17,19,23,31,43,61,...}. For n>1 A107036[n] = A000978[n] Numbers n such that (2^n + 1)/3 is prime. - Alexander Adamchuk (alex(AT)kolmogorov.com), Oct 10 2006

Cf. A001045, A107036, A000978, A000979.

Sequence in context: A055511 A105236 A144467 this_sequence A059242 A004203 A088884

Adjacent sequences: A049880 A049881 A049882 this_sequence A049884 A049885 A049886

nonn

Judson D. Neer (judson(AT)poboxes.com)