%I #20 Dec 12 2023 08:52:07
%S 1,2,1,-4,1,2,1,-4,1,2,1,-4,1,2,1,-4,1,2,1,-4,1,2,1,-4,1,2,1,-4,1,2,1,
%T -4,1,2,1,-4,1,2,1,-4,1,2,1,-4,1,2,1,-4,1,2,1,-4,1,2,1,-4,1,2,1,-4,1,
%U 2,1,-4,1,2,1,-4,1,2,1,-4,1,2,1,-4
%N Period 4: repeat [1, 2, 1, -4].
%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-1,-1,-1).
%F G.f.: 1/(1+x)+3x/(1+x^2). a(n) = (-1)^n+3*A056594(n-1). [_R. J. Mathar_, Oct 30 2008]
%F From _Wesley Ivan Hurt_, Jul 09 2016: (Start)
%F a(n) + a(n-1) + a(n-2) + a(n-3) = 0 for n>2, a(n) = a(n-4) for n>3.
%F a(n) = cos(n*Pi) + 3*sin(n*Pi/2). (End)
%p seq(op([1, 2, 1, -4]), n=0..40); # _Wesley Ivan Hurt_, Jul 09 2016
%t PadRight[{}, 100, {1, 2, 1, -4}] (* _Wesley Ivan Hurt_, Jul 09 2016 *)
%o (Magma) &cat [[1, 2, 1, -4]^^30]; // _Wesley Ivan Hurt_, Jul 09 2016
%Y Cf. A056594, A109008.
%K sign,easy
%O 0,2
%A _Paul Curtz_, Oct 07 2007