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%I #22 Mar 29 2019 21:06:59
%S 3,5,3,3,5,3,3,7,17,3,11,3,0,7,109,5,11,5,3,17,5,103,5,7,3,21943,3,53,
%T 17,7,3,19,3,11,5,7,5,37,3,3,19,167,7,13,3,89,43
%N Least number k > 0 such that ((2n-1)^k + 1)/(2n) is prime, or 0 if no such prime exists.
%C All terms are odd primes.
%C a(49) > 143800. - _Robert Price_, Mar 29 2019
%H H. Dubner and T. Granlund, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL3/DUBNER/dubner.html">Primes of the Form (b^n+1)/(b+1)</a>, J. Integer Sequences, 3 (2000), #P00.2.7.
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Repunit.html">Repunit</a>.
%F a(n) = A084742(2n-1).
%t A126659[n_] := Module[{k = 1}, If[n == 14, Return[0]]; While[! PrimeQ[((2 n - 1)^k + 1)/(2 n)], k++]; k]; Table[A126659[n], {n, 2, 48}] (* _Robert Price_, Oct 29 2018 *)
%o (PARI) a(n) = {if (n==14, return(0)); my(k=3); while (! isprime(((2*n-1)^k + 1)/(2*n)), k = nextprime(k+1)); k;} \\ _Michel Marcus_, Nov 23 2018
%Y Cf. A084741, A084742, A125954, A125958.
%K hard,more,nonn
%O 2,1
%A _Alexander Adamchuk_, Feb 09 2007, Feb 10 2007
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