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 A074475 #9 Sep 08 2022 08:45:07
%S A074475 3,1,6,8,17,29,56,100,187,341,630,1156,2129,3913,7200,13240,24355,
%T A074475 44793,82390,151536,278721,512645,942904,1734268,3189819,5866989,
%U A074475 10791078,19847884,36505953,67144913,123498752,227149616,417793283
%N A074475 a(n) = Sum_{j=0..floor(n/2)} T(2*j + q), where T(n) are generalized tribonacci numbers (A001644) and q = n - 2*floor(n/2).
%C A074475 a(n) is the convolution of T(n) with the sequence (1,0,1,0,1,0,...).
%H A074475 G. C. Greubel, <a href="/A074475/b074475.txt">Table of n, a(n) for n = 0..1000</a>
%H A074475 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0,2,2,1).
%F A074475 a(n) = 2*a(n-2) + 2*a(n-3) + a(n-4), a(0)=3, a(1)=1, a(2)=6, a(3)=8.
%F A074475 G.f.: (3+x)/(1-2*x^2-2*x^3-x^4).
%t A074475 CoefficientList[Series[(3+x)/(1-2*x^2-2*x^3-x^4), {x, 0, 40}], x]
%t A074475 LinearRecurrence[{0,2,2,1},{3,1,6,8},40] (* _Harvey P. Dale_, Jul 08 2017 *)
%o A074475 (PARI) my(x='x+O('x^40)); Vec((3+x)/(1-2*x^2-2*x^3-x^4)) \\ _G. C. Greubel_, Apr 21 2019
%o A074475 (Magma) R<x>:=PowerSeriesRing(Integers(), 40); Coefficients(R!( (3+x)/(1-2*x^2-2*x^3-x^4) )); // _G. C. Greubel_, Apr 21 2019
%o A074475 (Sage) ((3+x)/(1-2*x^2-2*x^3-x^4)).series(x, 40).coefficients(x, sparse=False) # _G. C. Greubel_, Apr 21 2019
%Y A074475 Cf. A001644, A074331.
%K A074475 easy,nonn
%O A074475 0,1
%A A074475 Mario Catalani (mario.catalani(AT)unito.it), Aug 23 2002