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 A099508 #8 Sep 25 2021 19:18:34
%S A099508 0,1,1,3,3,8,9,21,25,54,68,139,183,358,489,924,1301,2390,3450,6194,
%T A099508 9126,16080,24093,41805,63507,108817,167188,283533,439689,739391,
%U A099508 1155384,1929504,3033991,5038089,7962758,13161067,20889012,34394215,54778872
%N A099508 A transform of the Jacobsthal numbers.
%C A099508 A transform of A001045 under the mapping g(x)->(1/(1+x^3))g(x/(1+x^3)).
%H A099508 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (1,2,-2,1,0,-1)
%F A099508 G.f.: x/(1-x-2x^2+2x^3-x^4+x^6); a(n)=a(n-1)+2a(n-2)-2a(n-3)+a(n-4)-a(n-6); a(n)=sum{k=0..floor(n/3), binomial(n-2k, k)(-1)^k*J(n-3k)}.
%t A099508 LinearRecurrence[{1,2,-2,1,0,-1},{0,1,1,3,3,8},40] (* _Harvey P. Dale_, Sep 25 2021 *)
%Y A099508 Cf. A099505.
%K A099508 easy,nonn
%O A099508 0,4
%A A099508 _Paul Barry_, Oct 20 2004