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 A126589 #11 Feb 16 2025 08:33:04
%S A126589 4,9,16,25,32,36,49,64,81,100,121,125,144,169,196,216,225,243,256,289,
%T A126589 324,343,361,400,441,484,529,576,625,676,729,784,841,900,961,1000,
%U A126589 1024,1089,1156,1225,1296,1369,1444,1521,1600,1681,1728,1764,1849,1936,2025
%N A126589 Numbers n>1 such that prime of the form (n^k-1)/(n-1) does not exist for k>2; or A128164(n) = 0.
%C A126589 Appears to be the union of the perfect squares k^2 (for k>1) and the prime powers p^k (for k>1) with some exceptions, such as 2^3, 3^3, 2^7, etc.
%C A126589 The perfect powers except those of the form n^(p^m) where p and (n^(p^(m+1))-1)/(n^(p^m)-1) are primes, p>2 and m>=1. - _Max Alekseyev_, Mar 09 2009
%H A126589 H. Dubner, <a href="http://dx.doi.org/10.1090/S0025-5718-1993-1185243-9">Generalized repunit primes</a>, Math. Comp., 61 (1993), 927-930.
%H A126589 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Repunit.html">Repunit</a>.
%e A126589 A128164 begins with offset 2: {3, 3, 0, 3, 3, 5, 3, 0, 19, 17, 3, 5, 3, 3, 0, 3, ...}. Thus a(1) = 4, a(2) = 9, a(3) = 16.
%Y A126589 Cf. A128164, A084738, A065854, A084740, A084741, A065507, A084742.
%K A126589 nonn
%O A126589 1,1
%A A126589 _Alexander Adamchuk_, Mar 13 2007
%E A126589 Extended by _Max Alekseyev_, Mar 09 2009