A180510 G.f.: (t^5 + 2*t^4 + t^3 + 2*t^2 + t) / (t^6 + t^5 - 2*t^4 - 5*t^3 - 2*t^2 + t + 1).

0, 1, 1, 2, 7, 5, 20, 27, 49, 106, 155, 331, 560, 1013, 1917, 3310, 6223, 11117, 20140, 36899, 66185, 121014, 218791, 396703, 721280, 1305025, 2371433, 4298618, 7796439, 14150029, 25652500, 46550531, 84427441, 153141122, 277824947, 503893035, 914114320, 1658100757, 3007674389, 5455918726, 9896444495, 17951959061, 32563657260

Offset

0,4

Comments

An example of a sextic divisibility sequence whose characteristic polynomial has degree 6 and a 12-element dihedral Galois group. This example has a field and polynomial discriminant of 98000, which is one of the smallest possible.

References

Found by Noam D. Elkies and described in an email from Elkies to R. K. Guy, Jan 18 2011

Links

Vincenzo Librandi, Table of n, a(n) for n = 0..300 (corrected by Ray Chandler, Jan 19 2019)

E. L. Roettger, H. C. Williams, and R. K. Guy, Some extensions of the Lucas functions, Number Theory and Related Fields: In Memory of Alf van der Poorten, Series: Springer Proceedings in Mathematics & Statistics, Vol. 43, J. Borwein, I. Shparlinski, W. Zudilin (Eds.) 2013.

Index to divisibility sequences

Index entries for linear recurrences with constant coefficients, signature (-1,2,5,2,-1,-1).

Formula

a(n) = -a(-n) for all n in Z. - Michael Somos, Dec 30 2022

Example

G.f. = x + x^2 + 2*x^3 + 7*x^4 + 5*x^5 + 20*x^6 + 27*x^7 + 49*x^8 + 106*x^9 + ... - Michael Somos, Dec 30 2022

Mathematica

CoefficientList[ Series[(x^5 + 2x^4 + x^3 + 2x^2 + x)/(x^6 + x^5 - 2x^4 - 5x^3 - 2x^2 + x + 1), {x, 0, 42}], x] (* Robert G. Wilson v, Jun 26 2011 *)
a[1] = 0; a[2] = 1; a[3] = 1; a[4] = 2; a[5] = 7; a[6] = 5; a[n_Integer] := a[n] = -a[n - 6] - a[n - 5] + 2 a[n - 4] + 5 a[n - 3] + 2 a[n - 2] - a[n - 1] (* Or *)
LinearRecurrence[{-1, 2, 5, 2, -1, -1}, {0, 1, 1, 2, 7, 5}, 43] (* Roger L. Bagula, Mar 16 2012 *)
a[ n_] := a[n] = Sign[n]*With[{m = Abs[n]}, If[ m<4, {0, 1, 1, 2}[[m+1]], -a[m-1] +2*a[m-2] +5*a[m-3] +2*a[m-4] -a[m-5] -a[m-6]]]; (* Michael Somos, Dec 30 2022 *)

Prog

(Maxima) makelist(coeff(taylor(x*(x^4+2*x^3+x^2+2*x+1)/(x^6+x^5-2*x^4-5*x^3-2*x^2+x+1), x, 0, n), x, n), n, 1, 42);  /* Bruno Berselli, Jun 05 2011 */
(PARI) Vec((x^5+2*x^4+x^3+2*x^2+x)/(x^6+x^5-2*x^4-5*x^3-2*x^2+x+1)+O(x^99)) \\ Charles R Greathouse IV, Jun 06 2011
(PARI) {a(n) = sign(n)*polcoeff((x^5 + 2*x^4 + x^3 + 2*x^2 + x)/(x^6 + x^5 - 2*x^4 - 5*x^3 - 2*x^2 + x + 1) + x*O(x^abs(n)), abs(n))}; /* Michael Somos, Dec 30 2022 */

Crossrefs

Cf. A001351, A001945, A005120, A006235.

Sequence in context: A135566 A100759 A205448 * A152555 A343780 A094360

Adjacent sequences: A180507 A180508 A180509 * A180511 A180512 A180513

Keyword

nonn,easy

Author

N. J. A. Sloane, Jan 20 2011

Status

approved