login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A087168 Expansion of (1 + 2*x)/(1 + 3*x + 4*x^2). 6
1, -1, -1, 7, -17, 23, -1, -89, 271, -457, 287, 967, -4049, 8279, -8641, -7193, 56143, -139657, 194399, -24569, -703889, 2209943, -3814273, 2603047, 7447951, -32756041, 68476319, -74404793, -50690897, 449691863, -1146312001, 1640168551, -335257649, -5554901257 (list; graph; refs; listen; history; text; internal format)
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

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) = (1/14)*i*sqrt(7)*(-3/2 - (1/2)*i*sqrt(7))^n - (1/14)*i*sqrt(7)*(-3/2 + (1/2)*i*sqrt(7))^n + (1/2)*(-3/2 + (1/2)*i*sqrt(7))^n + (1/2)*(-3/2 - (1/2)*i*sqrt(7))^n, with n>=0 and i=sqrt(-1). - Paolo P. Lava, Jun 12 2008

a(n) = -a(-1-n) * 2^(2*n+1) = A001607(2*n + 1) for all n in Z. - Michael Somos, Sep 19 2014

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)/(4x^2 + 3x + 1), {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 */

CROSSREFS

Cf. A001607.

Sequence in context: A144695 A125244 A070416 * A247560 A215824 A239210

Adjacent sequences:  A087165 A087166 A087167 * A087169 A087170 A087171

KEYWORD

easy,sign

AUTHOR

Mario Catalani (mario.catalani(AT)unito.it), Aug 22 2003

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified July 28 15:51 EDT 2021. Contains 346335 sequences. (Running on oeis4.)