A123176 Numbers n such that (2^p + 1)/3 is prime, where p is the n-th prime.

2, 3, 4, 5, 6, 7, 8, 9, 11, 14, 18, 22, 26, 31, 39, 43, 46, 65, 69, 126, 267, 380, 495, 762, 1285, 1304, 1364, 1479, 1697, 4469, 8135, 9193, 11065, 11902, 12923, 13103, 23396, 23642, 31850, 77509, 285228

Offset: 1

Author

Alexander Adamchuk, Oct 03 2006

Keywords

• more
• nonn

Comments

Also prime(a(n)) are the indices of prime Jacobsthal numbers (A001045) with prime indices. Primes in the Jacobsthal sequence are listed in A049883.

Links

• C. Caldwell's The Top Twenty, Wagstaff.

Crossrefs

Cf. A000978, A000979, A001045, A049883, A107036, A123214.

Programs

Mathematica

Select[Range[500],PrimeQ[(2^Prime[#]+1)/3]&] (* The program generates the first 23 terms of the sequence. To generate more, increase the Range constant. *) (* Harvey P. Dale, Mar 09 2022 *)

Formula

a(n) = A000720( A000978(n) ).

Extensions

Two more terms computed from A000978 by Max Alekseyev, Mar 03 2010