OFFSET
0,2
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..850
Index entries for linear recurrences with constant coefficients, signature (20,-80).
FORMULA
G.f.: (1-16*x)/(1-20*x+80*x^2).
a(n) = (10 - 2*sqrt(5))^n*(1/2 + 3*sqrt(5)/10) + (10 + 2*sqrt(5))^n*(1/2 - 3*sqrt(5)/10).
a(n) = 20*a(n-1) - 80*a(n-2); a(0)=1, a(1)=4. - Harvey P. Dale, Jun 04 2013
a(n) = 2^(2*n-1)*5^((n-1)/2)*(-sqrt(5)*(1+(-1)^n)*Fibonacci(n-2) - (1 - (-1)^n)*Lucas(n-2)). - G. C. Greubel, Apr 21 2023
MATHEMATICA
CoefficientList[Series[(1-16x)/(1-20x+80x^2), {x, 0, 30}], x] (* or *) LinearRecurrence[{20, -80}, {1, 4}, 30] (* Harvey P. Dale, Jun 04 2013 *)
PROG
(Magma) [n le 2 select 3*n-2 else 20*(Self(n-1) - 4*Self(n-2)): n in [1..41]]; // G. C. Greubel, Apr 21 2023
(SageMath)
def A099841(n): return 4^n*5^((n-1)/2)*(-sqrt(5)*((n-1)%2)*fibonacci(n-2) - (n%2)*lucas_number2(n-2, 1, -1))
[A099841(n) for n in range(41)] # G. C. Greubel, Apr 21 2023
CROSSREFS
KEYWORD
easy,sign
AUTHOR
Paul Barry, Oct 27 2004
STATUS
approved