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%I #32 Dec 25 2023 11:24:25
%S 1,5,4,20,16,17,1,5,4,20,16,17,1,5,4,20,16,17,1,5,4,20,16,17,1,5,4,20,
%T 16,17,1,5,4,20,16,17,1,5,4,20,16,17,1,5,4,20,16,17,1,5,4,20,16,17,1,
%U 5,4,20,16,17,1,5,4,20,16,17,1,5,4,20,16,17,1,5,4,20,16,17,1,5,4,20,16
%N a(n) = 5^n mod 21.
%H G. C. Greubel, <a href="/A070374/b070374.txt">Table of n, a(n) for n = 0..1000</a>
%H <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (1,0,-1,1).
%F From _R. J. Mathar_, Apr 20 2010: (Start)
%F a(n) = a(n-1) - a(n-3) + a(n-4).
%F G.f.: ( -1-4*x+x^2-17*x^3 ) / ( (x-1)*(1+x)*(x^2-x+1) ). (End)
%F From _G. C. Greubel_, Mar 13 2016: (Start)
%F a(n) = a(n-6).
%F E.g.f.: 7*cosh(x) + 14*sinh(x) - 6*exp(x/2)*cos(sqrt(3)*x/2) - 4*sqrt(3)*exp(x/2)*sin(sqrt(3)*x/2). (End)
%t PowerMod[5, Range[0, 50], 21] (* _G. C. Greubel_, Mar 13 2016 *)
%o (Sage) [power_mod(5,n,21) for n in range(0,83)] # _Zerinvary Lajos_, Nov 26 2009
%o (PARI) a(n)=lift(Mod(5,21)^n) \\ _Charles R Greathouse IV_, Mar 22 2016
%K nonn,easy
%O 0,2
%A _N. J. A. Sloane_, May 12 2002
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