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 A098575 #20 Jul 08 2024 21:45:47
%S A098575 1,1,1,1,3,7,13,21,35,63,117,213,379,671,1197,2149,3859,6911,12357,
%T A098575 22101,39563,70847,126845,227045,406371,727391,1302101,2330901,
%U A098575 4172443,7468767,13369293,23931621,42838835,76683583,137266917,245713493
%N A098575 a(n) = Sum_{k=0..floor(n/4)} C(n-2*k,2*k)*2^k.
%H A098575 Vincenzo Librandi, <a href="/A098575/b098575.txt">Table of n, a(n) for n = 0..1000</a>
%H A098575 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1,0,2).
%F A098575 G.f.: (1-x)/((1-x)^2-2*x^4).
%F A098575 a(n) = 2*a(n-1) - a(n-2) + 2*a(n-4).
%t A098575 CoefficientList[Series[(1-x)/((1-x)^2-2x^4),{x,0,40}],x] (* _Vincenzo Librandi_, Jun 17 2012 *)
%o A098575 (Magma) I:=[1, 1, 1, 1]; [n le 4 select I[n] else 2*Self(n-1)-Self(n-2)+2*Self(n-4): n in [1..40]]; // _Vincenzo Librandi_, Jun 17 2012
%o A098575 (PARI) x='x+O('x^30); Vec((1-x)/((1-x)^2-2*x^4)) \\ _G. C. Greubel_, Feb 03 2018
%Y A098575 Cf. A007909, A098576.
%K A098575 easy,nonn
%O A098575 0,5
%A A098575 _Paul Barry_, Sep 16 2004