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

1,4

Links

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

Formula

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

Example

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.

Mathematica

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

Prog

(PFGW & SCRIPT)
SCRIPT
DIM n, 0
DIM k
LABEL loop1
SET n, n+1
SET k, 0
LABEL loop2
SET k, k+1
PRP 2^k*3^n-1
IF ISPRP THEN GOTO loop1
GOTO loop2

Crossrefs

Cf. A003307.

Sequence in context: A111986 A368744 A291455 * A172216 A121490 A197293

Adjacent sequences: A248846 A248847 A248848 * A248850 A248851 A248852

Keyword

nonn

Author

Pierre CAMI, Dec 03 2014

Status

approved