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 A115634 #19 Dec 06 2024 17:59:52
%S A115634 1,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,
%T A115634 -3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,0,-3,
%U A115634 0,-3,0,-3,0
%N A115634 Expansion of (1-4*x^2)/(1-x^2).
%C A115634 Row sums of number triangle A115633.
%H A115634 G. C. Greubel, <a href="/A115634/b115634.txt">Table of n, a(n) for n = 0..5000</a>
%H A115634 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (0,1).
%F A115634 a(n) = 4*0^n - 3*(1 + (-1)^n)/2.
%F A115634 a(n) = Sum_{k=0..n} A115633(n, k).
%F A115634 From _G. C. Greubel_, Nov 23 2021: (Start)
%F A115634 a(n) = 1 if n = 0, otherwise a(n) = -A010674(n-1).
%F A115634 E.g.f.: 4 - 3*cosh(x). (End)
%t A115634 Join[{1}, -3*Mod[Range[100] -1, 2]] (* _G. C. Greubel_, Nov 23 2021 *)
%t A115634 CoefficientList[Series[(1-4x^2)/(1-x^2),{x,0,100}],x] (* or *) LinearRecurrence[{0,1},{1,0,-3},100] (* or *) PadRight[{1},100,{-3,0}] (* _Harvey P. Dale_, Dec 06 2024 *)
%o A115634 (Magma) [4*0^n -3*(1+(-1)^n)/2: n in [0..100]]; // _G. C. Greubel_, Nov 23 2021
%o A115634 (Sage) [1]+[-3*((n-1)%2) for n in (1..100)] # _G. C. Greubel_, Nov 23 2021
%Y A115634 Cf. A010674, A115633.
%K A115634 easy,sign
%O A115634 0,3
%A A115634 _Paul Barry_, Jan 27 2006