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%I #13 Mar 29 2015 19:11:42
%S 1,1,1,25,1,5,1,97,77,19,37
%N Least k such that Mersenne-prime(n)*2^k-1 is prime (A000668(n)*2^k-1), or 0 if no such k exists.
%C 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).
%C The sequence continues (>10000), 167, (>5000), (>5000), 1081, 371, 995, 909, 857, 33, (>150), (>150), ...
%t 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 *)
%Y Cf. A000043, A098471.
%K hard,nonn
%O 1,4
%A _Pierre CAMI_, Sep 09 2004
%E Edited by _N. J. A. Sloane_ and _Robert G. Wilson v_, Sep 11 2004
%E a(19)-a(21) from _Robert G. Wilson v_, Sep 11 2004
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