A130325 Least prime p such that 3*p*2^n-1 is prime.

2, 2, 2, 2, 5, 2, 2, 3, 11, 17, 2, 23, 29, 3, 5, 3, 43, 2, 7, 17, 19, 3, 11, 23, 13, 19, 19, 7, 43, 23, 71, 5, 19, 2, 59, 11, 59, 2, 47, 31, 89, 5, 2, 3, 19, 107, 11, 71, 59, 23, 29, 7, 61, 89, 2, 173, 11, 17, 37, 37, 31, 23, 19, 2, 199, 103, 59, 37, 139, 17, 67, 41, 43

Offset: 0

Author

Pierre CAMI, May 24 2007

Keywords

• nonn

Examples

			3*2*2^0-1=5 is prime so for n=0 p=2.
3*2*2^1-1=11 is prime so for n=1 p=2.
		

Links

• Pierre CAMI, Table of n, a(n) for n = 0..500

Crossrefs

Cf. A130326, A130327.

Programs

Mathematica

s={};Do[ i=0;Until[PrimeQ[3Prime[i]*2^n-1],i++];AppendTo[s,Prime[i]] ,{n,0,72}];s (* James C. McMahon, Feb 08 2025 *)

PARI

a(n) = my(p=2); while (!isprime(3*p*2^n-1), p=nextprime(p+1)); p; \\ Michel Marcus, Sep 23 2019