OFFSET
0,2
COMMENTS
For positive n, a(n) equals the 5^n times the permanent of the (2n) X (2n) tridiagonal matrix with 1/sqrt(5)'s along the main diagonal, and i's along the superdiagonal and the subdiagonal (where i is the imaginary unit). - John M. Campbell, Jul 08 2011
LINKS
FORMULA
G.f.: (1 + 5*x)/(1 + 9*x + 25*x^2).
a(n) = -9*a(n-1) - 25*a(n-2), a(0)=1, a(1)=-4.
a(n) = Sum_{k=0..n} binomial(n+k,2*k)*(-5)^(n-k).
MATHEMATICA
CoefficientList[Series[(1 + 5x)/(25x^2 + 9x + 1), {x, 0, 25}], x]
LinearRecurrence[{-9, -25}, {1, -4}, 30] (* Harvey P. Dale, Sep 10 2017 *)
CROSSREFS
KEYWORD
easy,sign
AUTHOR
Mario Catalani (mario.catalani(AT)unito.it), Aug 22 2003
STATUS
approved