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A250200 Least number k>1 such that (2n-1)^k - 2 is prime, or 0 if no such number exists.

%I A250200 #24 Jul 24 2017 12:46:39
%S A250200 0,2,2,2,2,4,2,2,6,2,2,24,7,2,2,3,2,2,2,4,4,2,11,2,2,8,4,2,12,4,2,2,8,
%T A250200 3,2,2,4,2,2,38,130,4,4,4,2,3,2,4,747,3,4,2,10,2,3,17,10,13,2,2,2,6,
%U A250200 42,2,3,2,6,2,10,2,4,4,2,16,50,3,9,2,22,25
%N A250200 Least number k>1 such that (2n-1)^k - 2 is prime, or 0 if no such number exists.
%H A250200 Robert Price, <a href="/A250200/b250200.txt">Table of n, a(n) for n = 1..143</a>
%t A250200 lst = {0}; For[n = 2, n ≤ 143, n++, For[k = 2, k >= 1, k++, If[PrimeQ[(2*n - 1)^k - 2], AppendTo[lst, k]; Break[]]]]; lst
%t A250200 lnk[n_]:=Module[{k=2,c=2n-1},While[!PrimeQ[c^k-2],k++];k]; Join[{0}, Array[ lnk,80,2]] (* _Harvey P. Dale_, Jul 24 2017 *)
%Y A250200 Cf. A084712, A084713, A084714, A079706, A138066, A255707.
%K A250200 nonn
%O A250200 1,2
%A A250200 _Robert Price_, Mar 02 2015