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 A309907 #22 Sep 05 2019 09:41:56
%S A309907 1,169,17689,1776889,177768889,17777688889,1777776888889,
%T A309907 177777768888889,17777777688888889,1777777776888888889,
%U A309907 177777777768888888889,17777777777688888888889,1777777777776888888888889,177777777777768888888888889,17777777777777688888888888889
%N A309907 a(n) is the square of the number consisting of one 1 and n 3's: (133...3)^2.
%C A309907 All terms are zeroless (elements of A052382).
%H A309907 Seiichi Manyama, <a href="/A309907/b309907.txt">Table of n, a(n) for n = 0..500</a>
%H A309907 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A309907 G.f.: -(40*x^2+58*x+1)/((x-1)*(100*x-1)*(10*x-1)).
%F A309907 a(n) = A097166(n)^2 = ((4*10^n-1)/3)^2. - _Seiichi Manyama_, Aug 23 2019
%p A309907 a:= n-> parse(cat(1, 3$n))^2:
%p A309907 seq(a(n), n=0..18);
%o A309907 (PARI) {a(n) = ((4*10^n-1)/3)^2} \\ _Seiichi Manyama_, Aug 23 2019
%Y A309907 Cf. A052382, A097166, A102807.
%K A309907 nonn,base,easy
%O A309907 0,2
%A A309907 _Alois P. Heinz_, Aug 21 2019