A098472 - OEIS

A098472

Least k such that Mersenne-prime(n)*2^k-1 is prime (A000668(n)*2^k-1), or 0 if no such k exists.

1, 1, 1, 25, 1, 5, 1, 97, 77, 19, 37

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OFFSET

1,4

COMMENTS

Conjecture: Mersenne-prime(12) is a Riesel prime (that is, all numbers k^2*M(12)-1 are composite for all k) and similarly for M(14) and M(15).

The sequence continues (>10000), 167, (>5000), (>5000), 1081, 371, 995, 909, 857, 33, (>150), (>150), ...

LINKS

Table of n, a(n) for n=1..11.

MATHEMATICA

mexp = {the list in A000043}; f[n_] := Block[{k = 1, mp = 2^mexp[[n]] - 1}, While[ !PrimeQ[ mp*2^k - 1] && k < 5000, k++ ]; If[k == 5000, 0, k]]; Do[ Print[ f[n]], {n, 20}] (* Robert G. Wilson v, Sep 11 2004 *)

CROSSREFS

Cf. A000043, A098471.

Sequence in context: A040641 A240981 A040640 * A040642 A040643 A040644

Adjacent sequences: A098469 A098470 A098471 * A098473 A098474 A098475

KEYWORD

hard,nonn

AUTHOR

Pierre CAMI, Sep 09 2004

EXTENSIONS

Edited by N. J. A. Sloane and Robert G. Wilson v, Sep 11 2004

a(19)-a(21) from Robert G. Wilson v, Sep 11 2004

STATUS

approved