This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A126659 #23 Feb 16 2025 08:33:04
%S A126659 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 A126659 17,7,3,19,3,11,5,7,5,37,3,3,19,167,7,13,3,89,43
%N A126659 Least number k > 0 such that ((2n-1)^k + 1)/(2n) is prime, or 0 if no such prime exists.
%C A126659 All terms are odd primes.
%C A126659 a(49) > 143800. - _Robert Price_, Mar 29 2019
%H A126659 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 A126659 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Repunit.html">Repunit</a>.
%F A126659 a(n) = A084742(2n-1).
%t A126659 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 A126659 (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 A126659 Cf. A084741, A084742, A125954, A125958.
%K A126659 hard,more,nonn
%O A126659 2,1
%A A126659 _Alexander Adamchuk_, Feb 09 2007, Feb 10 2007