A248849 Smallest k>0 such that 2^k*3^n-1 is a prime number.

1, 1, 1, 3, 2, 5, 1, 1, 4, 3, 20, 1, 4, 13, 2, 11, 3, 101, 12, 1, 10, 9, 1, 11, 7, 27, 1, 347, 11, 73, 4, 7, 52, 93, 1, 7, 51, 73, 46, 11, 8, 41, 4, 51, 2, 5, 30, 11, 10, 3, 280, 11, 7, 17, 14, 1, 32, 11, 5, 11, 19, 1, 20, 17, 22, 133, 6, 1

Offset: 1

Author

Pierre CAMI, Dec 03 2014

Keywords

• nonn

Examples

			2^1*3^1-1=5 prime so a(1)=1.
2^1*3^2-1=17 prime so a(2)=1.
2^1*3^3-1=53 prime so a(3)=1.
		

Links

• Pierre CAMI, Table of n, a(n) for n = 1..1500

Crossrefs

Cf. A003307.

Programs

Mathematica

Flatten[{1,Table[k=0; While[Not[PrimeQ[2^k*3^n-1]],k++]; k,{n,2,100}]}] (* Vaclav Kotesovec, Dec 05 2014 *)

Formula

a(n)=1 for n=A003307(i).