A119681 Odd numbers n such that 2n-1 is prime.

3, 7, 9, 15, 19, 21, 27, 31, 37, 45, 49, 51, 55, 57, 69, 75, 79, 87, 91, 97, 99, 115, 117, 121, 129, 135, 139, 141, 147, 157, 159, 169, 175, 177, 187, 195, 199, 201, 205, 211, 217, 225, 229, 231, 255, 261, 271, 279, 285, 289, 297, 301, 307, 309, 321, 327, 331

Offset: 1

Author

Roger L. Bagula, Jun 12 2006

Keywords

• nonn
• easy

Comments

a(k) appears in the o.g.f. for floor(A002144(k)*n^2/4), for k >=1: x*(b(k)*(1 + x^2) + a(k)*x)/((1 - x)^3*(1 + x)), together with b(k) = (A002144(k)-1)/4 = A005098(k). - Wolfdieter Lang, Aug 07 2013

Programs

Mathematica

Select[Range[1,331,2],PrimeQ[2#-1]&] (* Harvey P. Dale, Feb 20 2013 *)

PARI

is(n)=n%2 && isprime(2*n-1) \\ Charles R Greathouse IV, Jun 06 2017

Formula

a(n) = (A002144(n) + 1)/2, n >= 1. - Wolfdieter Lang, Aug 07 2013

Extensions

Edited by N. J. A. Sloane, Sep 07 2006