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 A271894 #9 Jul 14 2024 18:52:50
%S A271894 1,3,7,20,50,138,356,964,2524,6768,17848,47624,126032,335472,889328,
%T A271894 2364352,6273184,16667808,44242240,117516608,311995328,828603648,
%U A271894 2200088960,5842620544,15513975040,41197898496,109395973888,290499691520,771395887616
%N A271894 Expansion of (1+x-3*x^2) / ( 1-2*x-4*x^2+6*x^3 ).
%C A271894 Sum of all second elements at level n of the TRIP-Stern sequence corresponding to the permutation triple (e,13,23)
%H A271894 Ilya Amburg, Krishna Dasaratha, Laure Flapan, Thomas Garrity, Chansoo Lee, Cornelia Mihaila, Nicholas Neumann-Chun, Sarah Peluse, Matthew Stoffregen, <a href="http://arxiv.org/abs/1509.05239">Stern Sequences for a Family of Multidimensional Continued Fractions: TRIP-Stern Sequences</a>, arXiv:1509.05239 [math.CO], 2015, Section 7.2.1
%H A271894 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (2,4,-6).
%F A271894 a(n) >= 2^n.
%t A271894 CoefficientList[Series[(1+x-3x^2)/(1-2x-4x^2+6x^3),{x,0,30}],x] (* or *) LinearRecurrence[{2,4,-6},{1,3,7},30] (* _Harvey P. Dale_, Jul 14 2024 *)
%o A271894 (PARI) x='x+O('x^99); Vec((1+x-3*x^2)/(1-2*x-4*x^2+6*x^3)) \\ _Altug Alkan_, Apr 16 2016
%Y A271894 Cf. A271486 (maximum at level n).
%K A271894 nonn,easy
%O A271894 0,2
%A A271894 _R. J. Mathar_, Apr 16 2016