A098555 Least k such that k*Mersenne-prime(n)-1 is prime.

1, 2, 2, 6, 2, 14, 2, 90, 18, 80, 230, 80, 354, 84, 84, 1782, 450, 828, 2562, 2502, 6882, 9288, 890, 16022, 11088, 31034, 7272, 92372

Offset: 1

Author

Pierre CAMI, Sep 14 2004

Keywords

• hard
• nonn

Crossrefs

Cf. A000043, A000668, A098556.

Programs

Mathematica

mexp = {the list in A000043}; f[n_] := Block[{k = 2, mp = 2^mexp[[n]] - 1}, While[ !PrimeQ[mp*k - 1] && k < 10000, k++ ]; If[k == 10000, 0, k]]; Do[ Print[ f[n]], {n, 22}] (* Robert G. Wilson v, Sep 17 2004 *)

Extensions

More terms from Pierre CAMI, Jan 17 2005

a(1)=1, a(23)=890, a(27)=7272 corrected, other terms through a(28)=92372 verified by Ray Chandler, Apr 16 2009