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 A168490 #17 May 10 2025 09:15:57
%S A168490 1,1,2,6,24,112,560,2888,15136,80160,427968,2300736,12445440,67702272,
%T A168490 370205184,2033976960,11224014336,62186741248,345825348608,
%U A168490 1929744008192,10802203119616,60644473282560,341383505977344,1926554113200128,10897582426832896,61775951537520640,350901756369403904
%N A168490 Sequence with Hankel transform equal to 2^floor(n^2/2).
%C A168490 Hankel transform is A099202 (a trivial Somos-4 sequence linked to y^2 = 1-12*x+44*x^2-48*x^3).
%H A168490 Vincenzo Librandi, <a href="/A168490/b168490.txt">Table of n, a(n) for n = 0..300</a>
%F A168490 G.f.: 1/(1-x/(1-x/(1-2*x/(1-2*x/(1-x/(1-x/(1-2*x/(1-2*x/(1-x/(1-x/(1-2*x/(1-... (continued fraction);
%F A168490 G.f.: 1/(1-x-x^2/(1-3*x-4*x^2/(1-3*x-x^2/(1-3*x-4*x^2/(1-3*x-x^2/(1-3*x-4*x^2/(1-... (continued fraction);
%F A168490 G.f.: (1-2*x-sqrt((1-2*x)*(1-10*x+24*x^2)))/(4*x*(1-2*x)).
%F A168490 Recurrence: (n+1)*a(n) = 4*(3*n-1)*a(n-1) - 4*(11*n-17)*a(n-2) + 24*(2*n-5)*a(n-3). - _Vaclav Kotesovec_, Oct 20 2012
%F A168490 a(n) ~ 2^(n-5/2)*3^(n+1)/(sqrt(Pi)*n^(3/2)). - _Vaclav Kotesovec_, Oct 20 2012
%F A168490 a(n) = Sum_{k=0..n} A168511(n,k)*2^(n-k). - _Philippe Deléham_, Mar 19 2013
%t A168490 CoefficientList[Series[(1-2*x-Sqrt[(1-2*x)(1-10*x+24*x^2)])/(4x*(1-2*x)), {x, 0, 20}], x] (* _Vaclav Kotesovec_, Oct 20 2012 *)
%K A168490 easy,nonn
%O A168490 0,3
%A A168490 _Paul Barry_, Nov 27 2009