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 A118406 #5 Jul 29 2021 17:13:31
%S A118406 1,2,2,4,4,8,14,28,52,104,206,412,820,1640,3278,6556,13108,26216,
%T A118406 52430,104860,209716,419432,838862,1677724,3355444,6710888,13421774,
%U A118406 26843548,53687092,107374184,214748366,429496732,858993460,1717986920,3435973838
%N A118406 Unsigned row sums of triangle A118404.
%H A118406 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (2,0,0,1,-2).
%F A118406 G.f.: A(x) = (1 - 2*x^2 - 5*x^4)/(1-x^4)/(1-2*x).
%t A118406 CoefficientList[Series[(1-2x^2-5x^4)/(1-x^4)/(1-2x),{x,0,40}],x] (* or *) LinearRecurrence[{2,0,0,1,-2},{1,2,2,4,4},40] (* _Harvey P. Dale_, Jul 29 2021 *)
%o A118406 (PARI) {a(n)=polcoeff((1 - 2*x^2 - 5*x^4)/(1-x^4)/(1-2*x+x*O(x^n)),n,x)}
%Y A118406 Cf. A118404, A118405.
%K A118406 nonn
%O A118406 0,2
%A A118406 _Paul D. Hanna_, Apr 27 2006