A128979 Least exponent k such that p_n*(2^k) - 1 is prime.

1, 1, 2, 1, 2, 3, 2, 1, 4, 4, 1, 1, 2, 7, 4, 2, 12, 3, 5, 2, 7, 1, 2, 4, 1, 10, 3, 10, 9, 8, 25, 2, 2, 1, 4, 5, 1, 3, 4, 2, 8, 3, 226, 3, 2, 1, 1, 3, 2, 1, 4, 4, 11, 6, 4, 2, 8, 1, 5, 2, 11, 2, 1, 26, 3, 6, 1, 1, 18, 3, 4, 4, 1, 7, 1, 2, 20, 5, 10, 3, 4, 7, 2, 3, 1, 6, 112, 9, 10, 7, 2, 12, 5, 46, 1, 2, 8

Offset: 1

Author

Pierre CAMI and Robert G. Wilson v, Feb 16 2007

Keywords

• nonn

Comments

Supposedly the difference from A101050 is that the k here are required to be strictly positive (nonzero positive). - R. J. Mathar, Dec 13 2008

Links

• Pierre CAMI and Robert G. Wilson v, Table of n, a(n) for n = 1..119.

Crossrefs

Cf. A000040, A101050, A126715.

Programs

Mathematica

f[n_] := Block[{k = 1, p = Prime@n}, While[ !PrimeQ[p*2^k - 1], k++ ]; k]; Array[f, 97]