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 A139800 #13 Jan 02 2024 08:59:53 %S A139800 1,0,0,0,2,2,4,8,18,34,68,136,274,546,1092,2184,4370,8738,17476,34952, %T A139800 69906,139810,279620,559240,1118482,2236962,4473924,8947848,17895698, %U A139800 35791394,71582788,143165576,286331154,572662306,1145324612,2290649224 %N A139800 a(n)=a(n-1)+a(n-2)+a(n-3)+2a(n-4). %H A139800 <a href="/index/Rec">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,1,2). %F A139800 O.g.f: (-1+x+x^2+x^3)/((2*x-1)(1+x)(1+x^2)). a(n)=2^n/15+(-1)^n/3+A112030(n)/5. - _R. J. Mathar_, Jun 12 2008 %K A139800 nonn %O A139800 0,5 %A A139800 _Paul Curtz_, May 22 2008 %E A139800 More terms from _R. J. Mathar_, Jun 12 2008