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 A026657 #16 Feb 02 2024 08:56:09
%S A026657 1,3,8,18,40,84,182,378,814,1686,3626,7506,16138,33402,71810,148626,
%T A026657 319522,661314,1421714,2942514,6325906,13092690,28147058,58255794,
%U A026657 125240050,259208562,557254322,1153345842,2479497394
%N A026657 a(n) = Sum_{i=0..n, j=0..n} A026648(i,j).
%H A026657 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,4,-4,2,-2).
%F A026657 G.f.: [(1+2x)(1+x^2)]/[(1-x)(1-4x^2-2x^4)]. - _Ralf Stephan_, Apr 30 2004
%t A026657 LinearRecurrence[{1, 4, -4, 2, -2}, {1, 3, 8, 18, 40}, 50] (* _Paolo Xausa_, Feb 02 2024 *)
%Y A026657 Cf. A026648.
%K A026657 nonn
%O A026657 0,2
%A A026657 _Clark Kimberling_