## Tokyo Journal of Mathematics

### On Convergents of Certain Values of Hyperbolic Functions Formed from Diophantine Equations

#### Abstract

Let $\xi=\sqrt{v/u}\tanh(uv)^{-1/2}$, where $u$ and $v$ are positive integers, and let $\eta=|h(\xi)|$, where $h(t)$ is a non-constant rational function with algebraic coefficients. We compute upper and lower bounds for the approximation of certain values $\eta$ of hyperbolic functions by rationals $x/y$ such that $x$ and $y$ satisfy Diophantine equations. We show that there are infinitely many coprime integers $x$ and $y$ such that $|y\eta-x|\ll\log\log y/\log y$ and a Diophantine equation holds simultaneously relating $x$ and $y$ and some integer $z$. Conversely, all positive integers $x$ and $y$ with $y\ge c_0$ solving the Diophantine equation satisfy $|y\eta-x|\gg\log\log y/\log y$.

#### Article information

Source
Tokyo J. Math., Volume 36, Number 1 (2013), 239-251.

Dates
First available in Project Euclid: 22 July 2013

https://projecteuclid.org/euclid.tjm/1374497522

Digital Object Identifier
doi:10.3836/tjm/1374497522

Mathematical Reviews number (MathSciNet)
MR3112386

Zentralblatt MATH identifier
1283.11103

#### Citation

CHAICHANA, Tuangrat; KOMATSU, Takao; LAOHAKOSOL, Vichian. On Convergents of Certain Values of Hyperbolic Functions Formed from Diophantine Equations. Tokyo J. Math. 36 (2013), no. 1, 239--251. doi:10.3836/tjm/1374497522. https://projecteuclid.org/euclid.tjm/1374497522

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