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 A100136 #14 Oct 10 2021 22:59:11
%S A100136 1,1,1,1,1,1,4,13,31,61,106,169,262,424,748,1417,2749,5251,9709,17395,
%T A100136 30553,53434,94285,168859,306283,558742,1017895,1844044,3320044,
%U A100136 5952472,10660177,19119385,34383781,61985497,111884665,201938701,364128136
%N A100136 a(n) = Sum_{k=0..floor(n/6)} C(n-3k,3k) * 3^k.
%H A100136 Seiichi Manyama, <a href="/A100136/b100136.txt">Table of n, a(n) for n = 0..1000</a>
%H A100136 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1,0,0,3).
%F A100136 G.f.: (1-x)^2/((1-x)^3 - 3x^6).
%F A100136 a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) + 3*a(n-6).
%Y A100136 Cf. A098576, A100134, A100135.
%K A100136 easy,nonn
%O A100136 0,7
%A A100136 _Paul Barry_, Nov 06 2004