A104060 Least k such that k*(k+1)*2^n-1 is prime.

1, 1, 2, 1, 2, 1, 9, 4, 3, 2, 17, 1, 5, 7, 9, 1, 2, 1, 17, 11, 9, 12, 20, 14, 24, 6, 9, 9, 41, 1, 14, 3, 2, 23, 9, 3, 2, 17, 59, 5, 3, 2, 30, 11, 21, 4, 39, 21, 41, 6, 32, 4, 3, 2, 72, 10, 39, 9, 72, 1, 36, 3, 2, 14, 17, 13, 84, 10, 15, 4, 122, 5, 6, 3, 2, 10, 41, 15, 5, 13, 5, 13, 90, 9, 38, 32

Offset

1,3

Comments

When k(n)=1 n+1 is prime and the prime is a Mersenne-prime when k(n)=3 then k(n+1)=2 and same prime for n and n+1

Links

Table of n, a(n) for n=1..86.

Example

1*2*2^1-1=3 prime so k(1)=1
1*2*2^2-1=7 prime so k(2)=1
2*3*2^3-1=47 prime so k(3)=2
9*10*2^7-1=11519 prime so k(7)=9

Mathematica

lk[n_]:=Module[{k=1, c=2^n}, While[!PrimeQ[k(k+1)c-1], k++]; k]; Array[lk, 90] (* Harvey P. Dale, Aug 06 2017 *)

Crossrefs

Sequence in context: A260897 A342920 A066772 * A062347 A124781 A124151

Adjacent sequences: A104057 A104058 A104059 * A104061 A104062 A104063

Keyword

nonn

Author

Pierre CAMI, Mar 31 2005

Extensions

Reformulated the definition - R. J. Mathar, Nov 13 2009

Status

approved