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

1,1

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

Links

Table of n, a(n) for n=1..57.

Formula

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

Mathematica

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

Prog

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

Crossrefs

Sequence in context: A026139 A328012 A292911 * A197625 A190811 A104177

Adjacent sequences: A119678 A119679 A119680 * A119682 A119683 A119684

Keyword

nonn,easy

Author

Roger L. Bagula, Jun 12 2006

Extensions

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

Status

approved