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 A081209 #18 Jul 16 2025 08:17:22
%S A081209 1,0,3,20,205,2604,39991,720600,14913081,348678440,9090909091,
%T A081209 261535698060,8230246567621,281241170407092,10371206370520815,
%U A081209 410525522232055664,17361641481138401521,781282469559318055056,37275544492386193492507,1879498672877297909667780
%N A081209 a(n) = Sum_{k=0..n} (-1)^(n-k)*n^k.
%C A081209 In base n, a(n) has n digits, which are (beginning from the left): n-1, 0, n-1, 0, n-1, 0, and so on, except that if n is even the rightmost digit is 1 instead of 0. For example, a(6) in base 6 is 505051, and a(7) in base 7 is 6060606. - _Mathew Englander_, Oct 19 2020
%H A081209 Seiichi Manyama, <a href="/A081209/b081209.txt">Table of n, a(n) for n = 0..386</a>
%F A081209 a(n) = (n^(n+1)+(-1)^n)/(n+1).
%t A081209 a[n_]:=(n^(n+1)+(-1)^n)/(n+1); Array[a,20,0] (* _Stefano Spezia_, Jul 15 2025 *)
%o A081209 (PARI) {a(n) = (-1)^n*sum(k=0, n, (-n)^k)} \\ _Seiichi Manyama_, Sep 12 2019
%Y A081209 Cf. A031973.
%K A081209 nonn,easy
%O A081209 0,3
%A A081209 _Vladeta Jovovic_, Apr 17 2003