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 A128979 Least exponent k such that p_n*(2^k) - 1 is prime. 2
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 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. MATHEMATICA f[n_] := Block[{k = 1, p = Prime@n}, While[ !PrimeQ[p*2^k - 1], k++ ]; k]; Array[f, 97] CROSSREFS Cf. A000040, A126715. Sequence in context: A337618 A048685 A101050 * A332509 A336157 A190167 Adjacent sequences:  A128976 A128977 A128978 * A128980 A128981 A128982 KEYWORD nonn AUTHOR Pierre CAMI and Robert G. Wilson v, Feb 16 2007 STATUS approved

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Last modified August 1 22:36 EDT 2021. Contains 346408 sequences. (Running on oeis4.)