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 A123102 #15 Aug 07 2022 07:39:14
%S A123102 1,0,1,4,5,12,29,56,121,264,553,1180,2525,5364,11429,24368,51889,
%T A123102 110544,235537,501748,1068917,2277276,4851437,10335464,22018729,
%U A123102 46908504,99933625,212898316,453557453,966256644,2058509045,4385438048
%N A123102 a(0)=1, a(1)=0, a(2)=1, a(n) = a(n-1) + a(n-2) + 3*a(n-3).
%H A123102 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,3).
%F A123102 a(n) + a(n+1) = A099213(n+1).
%F A123102 G.f.: (1-x)/(1-x-x^2-3*x^3).
%F A123102 If p[1]=0, p[2]=1, p[i]=4, (i>2), and if A is Hessenberg matrix of order n defined by: A[i,j]=p[j-i+1], (i<=j), A[i,j]=-1, (i=j+1), and A[i,j]=0 otherwise, then, for n>=1, a(n)=det A. - _Milan Janjic_, May 02 2010
%t A123102 LinearRecurrence[{1,1,3},{1,0,1},40] (* _Harvey P. Dale_, May 04 2018 *)
%o A123102 (PARI) Vec((1-x)/(1-x-x^2-3*x^3) + O(x^40)) \\ _Michel Marcus_, Aug 07 2022
%Y A123102 Cf. A099213.
%K A123102 nonn,easy
%O A123102 0,4
%A A123102 _Philippe Deléham_, Sep 27 2006
%E A123102 Corrected by _T. D. Noe_, Nov 07 2006