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February  2012, 32(2): 587-604. doi: 10.3934/dcds.2012.32.587

Rational periodic sequences for the Lyness recurrence

A. Gasull 1, , Víctor Mañosa 2, and  Xavier Xarles 3,

1. 

Dept. de Matemµatiques, Universitat Autónoma de Barcelona, Edifici C, 08193-Bellaterra, Barcelona, Spain
2. 

Dept. de Matemàtica Aplicada III (MA3), Control, Dynamics and Applications Group (CoDALab), Universitat Politècnica de Catalunya (UPC), Colom 1, 08222 Terrassa, Spain
3. 

Dept. de Matemàtiques, Universitat Autònoma de Barcelona, Edifici C, 08193 Bellaterra, Barcelona, Spain

Received  September 2010 Revised  December 2010 Published  September 2011

Consider the celebrated Lyness recurrence $ x_{n+2}=(a+x_{n+1})/x_{n}$ with $a\in\mathbb{Q}$. First we prove that there exist initial conditions and values of $a$ for which it generates periodic sequences of rational numbers with prime periods $1,2,3,5,6,7,8,9,10$ or $12$ and that these are the only periods that rational sequences $\{x_n\}_n$ can have. It is known that if we restrict our attention to positive rational values of $a$ and positive rational initial conditions the only possible periods are $1,5$ and $9$. Moreover 1-periodic and 5-periodic sequences are easily obtained. We prove that for infinitely many positive values of $a,$ positive 9-period rational sequences occur. This last result is our main contribution and answers an open question left in previous works of Bastien & Rogalski and Zeeman. We also prove that the level sets of the invariant associated to the Lyness map is a two-parameter family of elliptic curves that is a universal family of the elliptic curves with a point of order $n, n\ge5,$ including $n$ infinity. This fact implies that the Lyness map is a universal normal form for most birational maps on elliptic curves.
Keywords: rational points over elliptic curves, Lyness difference equations, universal family of elliptic curves., periodic points.
Mathematics Subject Classification: Primary: 39A20; Secondary: 39A11, 14H5.
Citation: A. Gasull, Víctor Mañosa, Xavier Xarles. Rational periodic sequences for the Lyness recurrence. Discrete & Continuous Dynamical Systems - A, 2012, 32 (2) : 587-604. doi: 10.3934/dcds.2012.32.587
References:
[1]

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G. Bastien and M. Rogalski, Global behavior of the solutions of Lyness' difference equation $u_{n+2}u_n=u_{n+1}+a$,, J. Difference Equations Appl., 10 (2004), 977.  doi: 10.1080/10236190410001728104.  Google Scholar

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F. Beukers and R. Cushman, Zeeman's monotonicity conjecture,, J. Differential Equations, 143 (1998), 191.  doi: 10.1006/jdeq.1997.3359.  Google Scholar

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W. Bosma, J. Cannon and C. Playoust, The Magma algebra system. I. The user language,, J. Symbolic Comput., 24 (1997), 235.  doi: 10.1006/jsco.1996.0125.  Google Scholar

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H. Cohen, "Number Theory. Volume I: Tools and Diophantine Equations,", Graduate Texts in Mathematics, 239 (2007).   Google Scholar

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J. Esch and T. D. Rogers, The screensaver map: Dynamics on elliptic curves arising from polygonal folding,, Discrete Comput. Geom., 25 (2001), 477.  doi: 10.1007/s004540010075.  Google Scholar

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D. Husemoller, "Elliptic Curves,", With an appendix by Ruth Lawrence, 111 (1987).   Google Scholar

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D. Jogia, J. A. G. Roberts and F. Vivaldi, An algebraic geometric approach to integrable maps of the plane,, J. Phys. A, 39 (2006), 1133.  doi: 10.1088/0305-4470/39/5/008.  Google Scholar

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I. Niven, H. S. Zukerman and H. L. Montgomery, "An Introduction to the Theory of Numbers,'', Fifth edition, (1991).   Google Scholar

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F. P. Rabarison, "Torsion et Rang des Courbes Elliptiques Définies sur les Corps de Nombres Algébriques,'', Thèse de doctorat, (2008).   Google Scholar

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W. Stein, et al., Sage: Open Source Mathematical Software (Version 4.0), The Sage Group, 2009., Available from: \url{http://www.sagemath.org/}., ().   Google Scholar

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J. Silverman, "Advanced Topics in the Arithmetic of Elliptic Curves,'', Graduate Texts in Mathematics, 151 (1994).   Google Scholar

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J. Silverman, "The Arithmetic of Elliptic Curves,'', Second edition, 106 (2009).   Google Scholar

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J. Silverman and J. Tate., "Rational Points on Elliptic Curves,'', Undergraduate Texts in Mathematics, (1992).   Google Scholar

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J. M. H. Olmsted, Rational values of trigonometric functions,, Amer. Math. Monthly, 52 (1945), 507.  doi: 10.2307/2304540.  Google Scholar

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E. C. Zeeman, Geometric unfolding of a difference equation,, Hertford College, (1996).   Google Scholar

show all references

References:
[1]

A. O. L. Atkin and F. Morain, Finding suitable curves for the elliptic curve method of factorization,, Math. Comp., 60 (1993), 399.  doi: 10.1090/S0025-5718-1993-1140645-1.  Google Scholar

[2]

E. Barbeau, B. Gelbord and S. Tanny, Periodicities of solutions of the generalized Lyness recursion,, J. Difference Equations Appl., 1 (1995), 291.  doi: 10.1080/10236199508808028.  Google Scholar

[3]

G. Bastien and M. Rogalski, Global behavior of the solutions of Lyness' difference equation $u_{n+2}u_n=u_{n+1}+a$,, J. Difference Equations Appl., 10 (2004), 977.  doi: 10.1080/10236190410001728104.  Google Scholar

[4]

A. Beauville, Les familles stables de courbes elliptiques sur P1 admettant quatre fibres singulières,, C. R. Acad. Sci. Paris, 294 (1982), 657.   Google Scholar

[5]

F. Beukers and R. Cushman, Zeeman's monotonicity conjecture,, J. Differential Equations, 143 (1998), 191.  doi: 10.1006/jdeq.1997.3359.  Google Scholar

[6]

W. Bosma, J. Cannon and C. Playoust, The Magma algebra system. I. The user language,, J. Symbolic Comput., 24 (1997), 235.  doi: 10.1006/jsco.1996.0125.  Google Scholar

[7]

H. Cohen, "Number Theory. Volume I: Tools and Diophantine Equations,", Graduate Texts in Mathematics, 239 (2007).   Google Scholar

[8]

J. E. Cremona, "Elliptic Curve Data," Web page maintained by W. Stein, University of Warwick., Available from: \url{http://www.warwick.ac.uk/staff/J.E.Cremona//ftp/data/}., ().   Google Scholar

[9]

A. Dujella, "Elliptic Curve Tables," Web page maintained by author, University of Zagreb., Available from: \url{http://web.math.hr/~duje/}., ().   Google Scholar

[10]

N. Elkies, Rational points near curves and small nonzero $|x^3 - y^2|$ via lattice reduction,, in, 1838 (2000), 33.   Google Scholar

[11]

J. Esch and T. D. Rogers, The screensaver map: Dynamics on elliptic curves arising from polygonal folding,, Discrete Comput. Geom., 25 (2001), 477.  doi: 10.1007/s004540010075.  Google Scholar

[12]

D. Husemoller, "Elliptic Curves,", With an appendix by Ruth Lawrence, 111 (1987).   Google Scholar

[13]

D. Jogia, J. A. G. Roberts and F. Vivaldi, An algebraic geometric approach to integrable maps of the plane,, J. Phys. A, 39 (2006), 1133.  doi: 10.1088/0305-4470/39/5/008.  Google Scholar

[14]

I. Niven, H. S. Zukerman and H. L. Montgomery, "An Introduction to the Theory of Numbers,'', Fifth edition, (1991).   Google Scholar

[15]

F. P. Rabarison, "Torsion et Rang des Courbes Elliptiques Définies sur les Corps de Nombres Algébriques,'', Thèse de doctorat, (2008).   Google Scholar

[16]

W. Stein, et al., Sage: Open Source Mathematical Software (Version 4.0), The Sage Group, 2009., Available from: \url{http://www.sagemath.org/}., ().   Google Scholar

[17]

J. Silverman, "Advanced Topics in the Arithmetic of Elliptic Curves,'', Graduate Texts in Mathematics, 151 (1994).   Google Scholar

[18]

J. Silverman, "The Arithmetic of Elliptic Curves,'', Second edition, 106 (2009).   Google Scholar

[19]

J. Silverman and J. Tate., "Rational Points on Elliptic Curves,'', Undergraduate Texts in Mathematics, (1992).   Google Scholar

[20]

J. M. H. Olmsted, Rational values of trigonometric functions,, Amer. Math. Monthly, 52 (1945), 507.  doi: 10.2307/2304540.  Google Scholar

[21]

E. C. Zeeman, Geometric unfolding of a difference equation,, Hertford College, (1996).   Google Scholar

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