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 A083423 #19 May 07 2024 03:22:59
%S A083423 1,2,9,18,81,162,729,1458,6561,13122,59049,118098,531441,1062882,
%T A083423 4782969,9565938,43046721,86093442,387420489,774840978,3486784401,
%U A083423 6973568802,31381059609,62762119218,282429536481,564859072962
%N A083423 a(n) = (5*3^n + (-3)^n)/6.
%H A083423 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (0,9).
%F A083423 From _Reinhard Zumkeller_, Mar 04 2011: (Start)
%F A083423 G.f.: (1+2x)/((1-3x)*(1+3x)).
%F A083423 E.g.f.: (5*exp(3x) + exp(-3x))/6.
%F A083423 a(n+3) = a(n+2)*a(n+1)/a(n). (End)
%F A083423 a(n) = 9*a(n-2). - _Vincenzo Librandi_, Mar 20 2011
%t A083423 LinearRecurrence[{0,9},{1,2},30] (* _Harvey P. Dale_, Dec 04 2019 *)
%Y A083423 Cf. A083424.
%K A083423 easy,nonn
%O A083423 0,2
%A A083423 _Paul Barry_, Apr 30 2003