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A070374
a(n) = 5^n mod 21.
1
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, 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, 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
OFFSET
0,2
FORMULA
From R. J. Mathar, Apr 20 2010: (Start)
a(n) = a(n-1) - a(n-3) + a(n-4).
G.f.: ( -1-4*x+x^2-17*x^3 ) / ( (x-1)*(1+x)*(x^2-x+1) ). (End)
From G. C. Greubel, Mar 13 2016: (Start)
a(n) = a(n-6).
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)
MATHEMATICA
PowerMod[5, Range[0, 50], 21] (* G. C. Greubel, Mar 13 2016 *)
PROG
(Sage) [power_mod(5, n, 21) for n in range(0, 83)] # Zerinvary Lajos, Nov 26 2009
(PARI) a(n)=lift(Mod(5, 21)^n) \\ Charles R Greathouse IV, Mar 22 2016
CROSSREFS
Sequence in context: A271409 A246326 A272506 * A057423 A327317 A215139
KEYWORD
nonn,easy
AUTHOR
N. J. A. Sloane, May 12 2002
STATUS
approved