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 A128415 #25 Sep 24 2022 11:44:50
%S A128415 1,-3,1,9,-31,57,-47,-87,449,-999,1201,393,-5983,16377,-25199,10089,
%T A128415 70529,-251943,473713,-413367,-654751,3617721,-8234159,10231593,
%U A128415 2241857,-47651943,133988401,-211357431,98118689
%N A128415 Expansion of (1-4x^2)/(1+3x+4x^2).
%C A128415 Row sums of number triangle A128414.
%H A128415 Vincenzo Librandi, <a href="/A128415/b128415.txt">Table of n, a(n) for n = 0..1000</a>
%H A128415 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (-3,-4).
%F A128415 For n>0, a(n) = (1/r)^n + (1/s)^n, with r = (-3-i*sqrt(7))/8 and s = (-3+i*sqrt(7))/8 the roots of 4x^2+3x+1. - _Ralf Stephan_, Jul 20 2013
%F A128415 a(n) = -3*a(n-1) - 4*a(n-2) for n > 2. - _Harry Richman_, May 05 2020
%t A128415 CoefficientList[Series[(1 - 4 x^2) / (1 + 3 x + 4 x^2), {x, 0, 40}], x] (* _Vincenzo Librandi_, Jul 20 2013 *)
%t A128415 LinearRecurrence[{-3,-4},{1,-3,1},40] (* _Harvey P. Dale_, Sep 24 2022 *)
%K A128415 easy,sign
%O A128415 0,2
%A A128415 _Paul Barry_, Mar 02 2007