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 A139814 #15 Jan 02 2024 09:00:02
%S A139814 0,1,3,7,11,23,47,95,187,375,751,1503,3003,6007,12015,24031,48059,
%T A139814 96119,192239,384479,768955,1537911,3075823,6151647,12303291,24606583,
%U A139814 49213167,98426335,196852667,393705335,787410671,1574821343,3149642683
%N A139814 a(n)=a(n-1)+a(n-2)+a(n-3)+2a(n-4); a(0)=0,a(1)=1,a(2)=3,a(3)=7.
%H A139814 <a href="/index/Rec">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,1,2).
%F A139814 O.g.f.: -x(1+2x+3x^2)/((2x-1)(1+x)(x^2+1)). a(n) = (-1)^(n+1)/3 +11*2^n/15 -2*(-1)^[n/2]*A000034(n)/5 . - _R. J. Mathar_, May 24 2008
%t A139814 LinearRecurrence[{1,1,1,2},{0,1,3,7},35] (* _Harvey P. Dale_, Apr 20 2011 *)
%Y A139814 Cf. A132429.
%K A139814 nonn,easy
%O A139814 0,3
%A A139814 _Paul Curtz_, May 23 2008
%E A139814 More terms from _R. J. Mathar_, May 24 2008