A117673 a(n) is the least k such that k*2*prime(n) + 1 is prime.

1, 1, 1, 2, 1, 2, 3, 5, 1, 1, 5, 2, 1, 2, 3, 1, 6, 3, 2, 4, 2, 2, 1, 1, 2, 3, 3, 3, 5, 1, 2, 1, 3, 2, 4, 3, 5, 2, 7, 1, 1, 3, 1, 2, 9, 2, 5, 6, 12, 6, 1, 1, 3, 1, 3, 3, 4, 3, 2, 1, 3, 1, 2, 3, 3, 13, 3, 5, 3, 5, 7, 1, 3, 2, 6, 6, 12, 3, 4, 2, 1, 5, 1, 2, 5, 1, 4, 15, 3, 6, 3, 4, 2, 1, 2, 3, 1, 16, 5, 9

Offset: 1

Author

Don Reble, Apr 25 2006

Keywords

• nonn

Comments

Iff a(n) = 1, prime(n) is a Sophie Germain prime, i.e., in A005384. - A.H.M. Smeets, Feb 01 2018

Examples

			a(8)=5 because 2*prime(8)=38 and 5*38 + 1 is prime.
		

Links

• Zak Seidov, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A016014, A035096, A074884.

Programs

Mathematica

Table[k := 1; While[ ! PrimeQ[2*k*Prime[n] + 1], k++ ]; k, {n, 1, 120}] (* Stefan Steinerberger, May 01 2006 *)

PARI

a(n) = {my(p=prime(n), k=1); while (!isprime(2*k*p+1), k++); k;} \\ Michel Marcus, Feb 12 2018