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  • I start learning about generating functions , so I ask , for example , what all the deduces that a generatingfunctionologist can make for a sequence like :

  • $x_{0}= c$ (some constant , say for example 1).

  • $x_{n+1}=x_{n}+\frac{1}{x_{n}}$

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See also: math.stackexchange.com/q/29777/1102 –  Aryabhata Sep 11 '11 at 14:49
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@Aryabhata, Grrrr... I spent the last 10 minutes of my life searching for this page, finally found it, and came back on this one to discover your comment. :-) –  Did Sep 11 '11 at 15:14
    
@Didier: Sorry :-) –  Aryabhata Sep 11 '11 at 20:55
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1 Answer

up vote 2 down vote accepted

Not sure that generating functions enter the picture here but the asymptotics of $(x_n)$ is clear: first prove that $x_n\to+\infty$, then note that $x_{n+1}^2=x_n^2+2+1/x_n^2$, and that this implies first that $x_n^2=2n+o(n)$ and then that $x_n^2=2n+\frac12\log n+o(\log n)$.

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