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 A099857 #21 Oct 01 2023 18:25:02
%S A099857 1,6,18,48,126,330,864,2262,5922,15504,40590,106266,278208,728358,
%T A099857 1906866,4992240,13069854,34217322,89582112,234529014,614004930,
%U A099857 1607485776,4208452398,11017871418,28845161856,75517614150,197707680594
%N A099857 Expansion of (1+3x+x^2)/(1-3x+x^2).
%C A099857 Hankel transform is 1, -18, -36, 0, 0, 0, 0, 0, 0, 0, 0, ... - _Philippe Deléham_, Dec 15 2011
%H A099857 Michael De Vlieger, <a href="/A099857/b099857.txt">Table of n, a(n) for n = 0..2391</a>
%H A099857 Hacène Belbachir, Soumeya Merwa Tebtoub, and László Németh, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL23/Nemeth/nemeth7.html">Ellipse Chains and Associated Sequences</a>, J. Int. Seq., Vol. 23 (2020), Article 20.8.5.
%H A099857 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (3, -1).
%F A099857 a(n) = 0^n + 6*Fibonacci(2*n).
%t A099857 CoefficientList[Series[(1+3x+x^2)/(1-3x+x^2),{x,0,30}],x] (* _Harvey P. Dale_, May 23 2015 *)
%Y A099857 Cf. A000045, A099856, A099858.
%K A099857 easy,nonn
%O A099857 0,2
%A A099857 _Paul Barry_, Oct 28 2004