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A176248 a(n) is the least odd multiple of 3, k, such that k*2^n-1 is a Sophie Germain prime. 1
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 (list; graph; refs; listen; history; text; internal format)
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: A229847 A196529 A110668 * A290159 A083562 A332860

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

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Last modified January 16 19:48 EST 2022. Contains 350376 sequences. (Running on oeis4.)