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 A109110 #11 Apr 27 2025 12:17:47
%S A109110 4,9,20,45,101,227,510,1146,2575,5786,13001,29213,65641,147494,331416,
%T A109110 744685,1673292,3759853,8448313,18983187,42654834,95844542,215360731,
%U A109110 483911170,1087338529,2443227497,5489882353,12335653674,27717962204
%N A109110 a(n) = 2a(n-1) + a(n-2) - a(n-3); a(0)=4, a(1)=9, a(2)=20.
%C A109110 Kekulé numbers for certain benzenoids.
%D A109110 S. J. Cyvin and I. Gutman, Kekulé structures in benzenoid hydrocarbons, Lecture Notes in Chemistry, No. 46, Springer, New York, 1988 (pp. 286, 288, K{S(n)})
%H A109110 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (2,1,-1)
%F A109110 G.f.: (4 + z - 2z^2)/(1 - 2z - z^2 + z^3).
%F A109110 a(n) = A052534(n+2). - _R. J. Mathar_, Feb 03 2014
%p A109110 a[0]:=4:a[1]:=9:a[2]:=20: for n from 3 to 32 do a[n]:=2*a[n-1]+a[n-2]-a[n-3] od: seq(a[n],n=0..32);
%t A109110 LinearRecurrence[{2,1,-1},{4,9,20},30] (* _Harvey P. Dale_, Apr 27 2025 *)
%K A109110 nonn,easy
%O A109110 0,1
%A A109110 _Emeric Deutsch_, Jun 19 2005