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 A109108 #12 Jan 04 2024 15:16:16
%S A109108 1,9,91,919,9281,93729,946571,9559439,96540961,974969049,9846231451,
%T A109108 99437283559,1004219067041,10141627953969,102420498606731,
%U A109108 1034346614021279,10445886638819521,105493213002216489
%N A109108 a(n) = 10a(n-1) + a(n-2), a(0)=1, a(1)=9.
%C A109108 Kekulé numbers for certain benzenoids.
%D A109108 S. J. Cyvin and I. Gutman, Kekulé structures in benzenoid hydrocarbons, Lecture Notes in Chemistry, No. 46, Springer, New York, 1988 (p. 284, K{Q_1(n)}).
%H A109108 Tanya Khovanova, <a href="http://www.tanyakhovanova.com/RecursiveSequences/RecursiveSequences.html">Recursive Sequences</a>
%H A109108 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (10,1).
%F A109108 a(n) = (1/2/sqrt(26))((sqrt(26)+4)(5+sqrt(26))^n+(sqrt(26)-4)(5-sqrt(26))^n).
%F A109108 G.f.: (1-z)/(1-10z-z^2).
%p A109108 a:=n->(1/2/sqrt(26))*((sqrt(26)+4)*(5+sqrt(26))^n+(sqrt(26)-4)*(5-sqrt(26))^n): seq(expand(a(n)),n=0..20);
%t A109108 LinearRecurrence[{10,1},{1,9},20] (* _Harvey P. Dale_, Jan 04 2024 *)
%Y A109108 First differences of A041041.
%K A109108 nonn,easy
%O A109108 0,2
%A A109108 _Emeric Deutsch_, Jun 19 2005