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 A049863 #25 Feb 04 2025 18:27:15
%S A049863 0,3,4,7,11,21,39,74,138,258,481,898,1676,3129,5841,10904,20355,37998,
%T A049863 70933,132415,247187,461439,861396,1608020,3001788,5603619,10460614,
%U A049863 19527460,36453089,68049183,127031520,237137411,442678728,826375119,1542644347,2879747377
%N A049863 a(n) = Sum{a(k): k=0,1,2,...,n-4,n-2,n-1}; a(n-3) is not a summand; initial terms are 0,3,4.
%H A049863 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2, 0, -1, 1).
%F A049863 a(n) = 2*a(n-1) - a(n-3) + a(n-4).
%F A049863 G.f.: (x^3+2x^2-3x)/(x^4-x^3+2x-1). - _Harvey P. Dale_, Apr 26 2011
%t A049863 LinearRecurrence[{2,0,-1,1},{0,3,4,7},40] (* or *) CoefficientList[ Series[(x^3+2x^2-3x)/(x^4-x^3+2x-1),{x,0,50}],x] (* _Harvey P. Dale_, Apr 26 2011 *)
%K A049863 nonn,easy
%O A049863 0,2
%A A049863 _Clark Kimberling_