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 A169725 #26 Sep 08 2022 08:45:49
%S A169725 12,70,532,4510,39772,355510,3192292,28708750,258313132,2324621350,
%T A169725 20921001652,188287243390,1694579876092,15251202941590,
%U A169725 137260778644612,1235346864312430,11118121348344652,100063090843700230,900567813719097172,8105110311849259870
%N A169725 a(n) = 3^(n-1)*(6*3^(n-1) + 5) + 1.
%H A169725 Vincenzo Librandi, <a href="/A169725/b169725.txt">Table of n, a(n) for n = 1..1000</a>
%H A169725 Alice V. Kleeva, <a href="/A169725/a169725a.jpg">Grid for this sequence</a>
%H A169725 Alice V. Kleeva, <a href="/A169725/a169725b.jpg">Illustration of initial terms</a>
%H A169725 Robert Munafo, <a href="http://www.mrob.com/pub/math/seq-a169720.html">Sequence A169720, and two others by Alice V. Kleeva</a>
%H A169725 Robert Munafo, <a href="/A169720/a169720.pdf">Sequence A169720, and two others by Alice V. Kleeva</a> [Cached copy, in pdf format, included with permission]
%H A169725 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (13,-39,27).
%F A169725 From _R. J. Mathar_, Apr 26 2010: (Start)
%F A169725 a(n) = 13*a(n-1) - 39*a(n-2) + 27*a(n-3).
%F A169725 G.f.: x*( -12 + 86*x - 90*x^2 ) / ( (x-1)*(3*x-1)*(9*x-1) ). (End)
%F A169725 E.g.f.: (2*exp(9*x) + 5*exp(3*x) + 3*exp(x) - 10)/3. - _Stefano Spezia_, Dec 25 2021
%p A169725 A169725 := proc(n)
%p A169725 3^(n-1)*(6*3^(n-1)+5)+1 ;
%p A169725 end proc: # _R. J. Mathar_, Jun 02 2016
%t A169725 Table[3^(n-1) (6 3^(n - 1) + 5) + 1, {n, 20}] (* or *) LinearRecurrence[{13, -39, 27}, {12, 70, 532}, 20] (* _Harvey P. Dale_, Aug 10 2011 *)
%t A169725 CoefficientList[Series[(-12 + 86 x - 90 x^2)/((x - 1) (3 x - 1) (9 x - 1)), {x, 0, 30}], x] (* _Vincenzo Librandi_, Dec 03 2012 *)
%o A169725 (Magma) I:=[12, 70, 532]; [n le 3 select I[n] else 13*Self(n-1) -39*Self(n-2) +27*Self(n-3): n in [1..30]]; // _Vincenzo Librandi_, Dec 03 2012
%Y A169725 Cf. A169720-A169727, A100702.
%K A169725 nonn,easy
%O A169725 1,1
%A A169725 Alice V. Kleeva (alice27353(AT)gmail.com), Jan 19 2010
%E A169725 G.f. adapted to the offset by _Vincenzo Librandi_, Dec 03 2012