A176248 a(n) is the least odd multiple of 3, k, such that k*2^n-1 is a Sophie Germain prime.

3, 3, 3, 15, 45, 3, 99, 45, 51, 141, 153, 177, 411, 45, 45, 267, 237, 75, 75, 207, 111, 111, 123, 159, 57, 375, 1419, 45, 291, 321, 489, 585, 525, 1623, 579, 45, 27, 1293, 1059, 255, 2265, 33, 465, 165, 405, 315, 315, 117, 411, 1725

Offset

1,1

Links

Pierre CAMI, Table of n, a(n) for n = 1..1073

Example

3*2^6 - 1 = 3*64 - 1 = 191 that is the 15th Sophie Germain prime, so a(6) = 3. - Bernard Schott, Sep 16 2019

Prog

(PARI) a(n) = my(k=1, p); while(!(isprime(p=3*k*2^n-1) && isprime(2*p+1)), k+=2); 3*k; \\ Michel Marcus, Sep 16 2019
(Magma) sol:=[]; for n in [1..50] do k:=3; while not IsPrime(k*2^n-1) or not IsPrime(2*(k*2^n-1)+1) do k:=k+6; end while; Append(~sol, k); end for; sol; // Marius A. Burtea, Sep 16 2019

Crossrefs

Cf. A005384 (Sophie Germain primes).

Sequence in context: A196529 A110668 A367773 * A290159 A356388 A083562

Adjacent sequences: A176245 A176246 A176247 * A176249 A176250 A176251

Keyword

nonn

Author

Pierre CAMI, Apr 13 2010

Extensions

Name, data and b-file corrected by Michel Marcus, Sep 16 2019

Status

approved