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 A131664 #15 Oct 18 2022 03:42:01
%S A131664 1,11,11,111,111,111,1111,1111,1111,1111,11111,11111,11111,11111,
%T A131664 11111,111111,111111,111111,111111,111111,111111,1111111,1111111,
%U A131664 1111111,1111111,1111111,1111111,1111111,11111111,11111111,11111111,11111111
%N A131664 A string of n 1's repeated n times.
%H A131664 Robert Israel, <a href="/A131664/b131664.txt">Table of n, a(n) for n = 1..10000</a>
%F A131664 G.f.: z/(1-z) * Sum_{n>=0} 10^n*z^(n*(n+1)/2).
%p A131664 seq(((10^n-1)/9)$n, n=1..10); # _Robert Israel_, Jul 08 2019
%o A131664 (Python)
%o A131664 from math import isqrt
%o A131664 def A131664(n): return (10**(isqrt(n<<3)+1>>1)-1)//9 # _Chai Wah Wu_, Oct 17 2022
%Y A131664 Cf. A000042, A002024.
%K A131664 nonn,base
%O A131664 1,2
%A A131664 _Paul Curtz_, Oct 03 2007