OFFSET
0,4
COMMENTS
For positive n, a(n) equals 2^n times the permanent of the (2n) X (2n) tridiagonal matrix with 1/sqrt(2)'s along the main diagonal, and i's along the superdiagonal and the subdiagonal (i is the imaginary unit). - John M. Campbell, Jul 08 2011
For n > 3, equals -1 times the determinant of the (n-2) X (n-2) matrix with 2^2's along the superdiagonal, 3^2's along the main diagonal, 4^2's along the subdiagonal, etc., and 0's everywhere else. - John M. Campbell, Dec 01 2011
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..1000
Taras Goy and Mark Shattuck, Determinants of Toeplitz-Hessenberg Matrices with Generalized Leonardo Number Entries, Ann. Math. Silesianae (2023). See p. 14.
Index entries for linear recurrences with constant coefficients, signature (-3,-4).
FORMULA
G.f.: (1+2*x)/(1+3*x+4*x^2).
a(n) = -3*a(n-1) - 4*a(n-2); a(0)=1, a(1)=-1.
a(n) = Sum_{k=0..n} C(n+k,2*k)*(-2)^(n-k).
a(n) = -a(-1-n) * 2^(2*n+1) = A001607(2*n + 1) for all n in Z. - Michael Somos, Sep 19 2014
a(n) = (-2)^(n-1)*(2*ChebyshevU(n-2, 3/4) - ChebyshevU(n-1, 3/4)). - G. C. Greubel, Jun 09 2022
EXAMPLE
G.f. = 1 - x - x^2 + 7*x^3 - 17*x^4 + 23*x^5 - x^6 - 89*x^7 + 271*x^8 + ...
MATHEMATICA
CoefficientList[Series[(1+2x)/(1+3x+4x^2), {x, 0, 30}], x]
Table[-Det[Array[Sum[KroneckerDelta[#1, #2+q]*(q+3)^2, {q, -1, n-2}] &, {n-2, n-2}]], {n, 4, 50}] (* John M. Campbell, Dec 01 2011 *)
LinearRecurrence[{-3, -4}, {1, -1}, 40] (* Harvey P. Dale, Apr 23 2014 *)
PROG
(Magma)
A087168:= func< n | &+[ Binomial(n+k, 2*k)*(-2)^(n-k): k in [0..n] ] >;
[A087168(n): n in [0..35]];
(PARI) {a(n) = real( (-1 - quadgen(-7))^n )}; /* Michael Somos, Sep 19 2014 */
(SageMath)
def A087168(n): return (-2)^(n-1)*(2*chebyshev_U(n-2, 3/4) -chebyshev_U(n-1, 3/4))
[A087168(n) for n in (0..50)] # G. C. Greubel, Jun 09 2022
CROSSREFS
KEYWORD
easy,sign
AUTHOR
Mario Catalani (mario.catalani(AT)unito.it), Aug 22 2003
STATUS
approved