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 A204847 #31 May 05 2021 14:06:43
%S A204847 1,11,111,101,11111,91,1111111,10001,333667,9091,11111111111,9901,
%T A204847 1111111111111,909091,90090991,100000001,11111111111111111,999001,
%U A204847 1111111111111111111,99009901,900900990991,826446281,11111111111111111111111,99990001,100001000010000100001
%N A204847 Primitive cofactor of n-th repunit A002275(n).
%C A204847 Except for a(1) = 1 and a(3) = 111, this is the Zsigmondy numbers for a = 10, b = 1: Zs(n, 10, 1) is the greatest divisor of 10^n - 1^n that is coprime to 10^m - 1^m for all positive integers m < n. The prime terms are called unique primes or unique period primes (A007615).
%C A204847 Differs from A019328 for n = 1, 9, 22, 27, 42, ... - _Jianing Song_, Apr 30 2018
%H A204847 Makoto Kamada, <a href="https://stdkmd.net/nrr/repunit">Factorizations of 11...11 (Repunit)</a>.
%H A204847 Samuel Yates, <a href="http://www.geocities.jp/ma85003/math/repdigit.pdf">Cofactors of repunits</a>, Journal of Recreational Mathematics, Vol. 8(2), pp. 99, 1975-76.
%H A204847 Samuel Yates, <a href="http://www.jstor.org/stable/2689643">The Mystique of Repunits</a>, Math. Mag. 51 (1978), 22-28.
%F A204847 Equals A002275(n)/(product of terms in n-th row of A204845).
%o A204847 (PARI) lista(nn) = {vf = []; vfs = []; for (n=1, nn, if (n==1, print1(n, ", "), f = factor((10^n-1)/9)[,1]; vkeep = []; for (k = 1, #f~, if (!vecsearch(vfs, f[k]), vkeep = concat(vkeep, f[k]));); print1(prod(j=1, #vkeep, vkeep[j]), ", "); vf = concat(vf, vkeep); vfs = Set(vf);););} \\ _Michel Marcus_, May 18 2018
%Y A204847 Cf. A002275, A019328, A102380, A204845, A204846.
%K A204847 nonn
%O A204847 1,2
%A A204847 _N. J. A. Sloane_, Jan 19 2012
%E A204847 a(11)-a(24) from _Jianing Song_, Apr 30 2018
%E A204847 a(25) from _Jinyuan Wang_, May 02 2021