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 A139763 #17 Dec 12 2023 08:35:54
%S A139763 1,2,3,4,11,22,43,84,171,342,683,1364,2731,5462,10923,21844,43691,
%T A139763 87382,174763,349524,699051,1398102,2796203,5592404,11184811,22369622
%N A139763 a(n)=a(n-1)+a(n-2)+a(n-3)+2a(n-4) with a(n)=n+1 for n<=3.
%H A139763 Harvey P. Dale, <a href="/A139763/b139763.txt">Table of n, a(n) for n = 0..1000</a>
%H A139763 <a href="/index/Rec">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,1,2)
%F A139763 a(n+1)-2a(n)=period 4:repeat 0, -1, -2, 3.
%F A139763 O.g.f.: (x-1)*(2*x^2+2*x+1)/((2*x-1)(1+x)(x^2+1)). a(n) = A056594(n+3)+((-1)^n+2^(n+1))/3. - _R. J. Mathar_, May 22 2008
%t A139763 LinearRecurrence[{1,1,1,2},{1,2,3,4},30] (* _Harvey P. Dale_, May 22 2021 *)
%K A139763 nonn
%O A139763 0,2
%A A139763 _Paul Curtz_, May 20 2008
%E A139763 More terms from _R. J. Mathar_, May 22 2008