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

Author

Pierre CAMI, Apr 13 2010

Keywords

• nonn
• changed

Examples

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

Links

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

Crossrefs

Cf. A005384 (Sophie Germain primes), A179658.

Programs

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

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

Extensions

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