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What are the closed-form inverses of the injective pieces of $x\sinh(x)$, $x\cosh(x)$, $x\tanh(x)$, $x\ \text{sech}(x)$, $x\coth(x)$, $x\ \text{csch}(x)$?

I assume these functions don't have inverses that are elementary functions.

Can the inverses be represented with help of Lambert W?

Closed-form inverses can give hints for properties and calculation of the inverses.

I ask here because I want to present in my answer the closed-form representations of the inverses I found.

Further answers and methods are welcome.

IV_
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  • You have many inverse functions/ requests in a single question. A slightly different $x\sec(x)$ inverse series is a Kepler equation with a “closed form” using “Kepler E”, but this function is not really anywhere else; likely the same is possible for $x,\text{csch}(x)$. Would you like an explicit series solution for an inverse of $x\cosh(x)$ and $x\sinh(x)$ even though they are not a closed form? – Тyma Gaidash Dec 19 '22 at 21:40
  • @TymaGaidash I wanted to point out the Leal-functions. We can take them as closed-form functions because some of their algebraic properties and their applicability for some other kinds of equations are presented in the cited article. – IV_ Dec 20 '22 at 21:50
  • Question to the downvoters: Why is my valuable question downvoted? The aim of my question is to popularize this type of previously unknown mathematical solutions to enrich mathematics. – IV_ Jan 08 '24 at 20:32

1 Answers1

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Elementary functions:

The function terms of the hyperbolic functions ($\sinh(x)$, $\cosh(x)$, $\tanh(x)$, $\text{sech}(x)$, $\coth(x)$, $\text{csch}(x)$) are irreducible rational expressions of $e^x$: Wikipedia: Hyperbolic functions - Exponential definitions. The function terms $x\ \text{hyp}(x)$, wherein $\text{hyp}$ is one of the hyperbolic functions, are irreducible rational expressions of both $x$ and $e^x$ therefore.
Liouville and Ritt proved that such kind of functions (over a complex domain without isolated points) don't have inverses that are elementary functions: How can we show that $A(z,e^z)$ and $A(\ln (z),z)$ have no elementary inverse?

Lambert W:

The defining equations for the inverse $x\ \text{hyp}(x)=y$ can be rearranged to polynomial equations of both $x$ and $e^x$ which are quadratic for $e^x$. These equations are not in a form to apply Lambert W.

Generalized Lambert W:

The equations $x\tanh(x)=y$ and $x\coth(x)$ can be solved in terms of Generalized Lambert W of [Mezö 2017], [Mezö/Baricz 2017], [Castle 2018]:

\begin{matrix} x\tanh(x)=y & & x\coth(x)=y\\ x\frac{e^{2x}-1}{e^{2x}+1}=y & & x\frac{e^{2x}+1}{e^{2x}-1}=y\\ \frac{x-y}{x+y}e^{2x}=1 & & \frac{x-y}{(x+y)}e^{2x}=-1\\ x=\frac{1}{2}W(^{+2y}_{-2y};1) & & x=\frac{1}{2}W(^{+2y}_{-2y};-1)\\ \end{matrix}

"Leal-functions":

The inverses of the functions mentioned in the question are presented in [Vazquez-Leal et al. 2020].

We can take them as closed-form functions because some of their algebraic properties and their applicability for some other kinds of equations are presented in the cited article.
$\ $

[Castle 2018] Castle, P.: Taylor series for generalized Lambert W functions. 2018

[Mezö 2017] Mezö, I.: On the structure of the solution set of a generalized Euler-Lambert equation. J. Math. Anal. Appl. 455 (2017) (1) 538-553

[Mezö/Baricz 2017] Mezö, I.; Baricz, A.: On the generalization of the Lambert W function. Transact. Amer. Math. Soc. 369 (2017) (11) 7917–7934 (On the generalization of the Lambert W function with applications in theoretical physics. 2015)

[Vazquez-Leal et al. 2020] Vazquez-Leal, H.; Sandoval-Hernandez, M. A.; Filobello-Ninoa, U.: The novel family of transcendental Leal-functions with applications to science and engineering. Heliyon 6 (2020) (11) e05418

IV_
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  • THE Thrones asks: I'm asking for IV_'s transformations in his answer. How did he transform from line 2 to 3? – IV_ Jan 08 '24 at 20:11
  • @THEThrones All elementary standard functions (trigonometric functions, inverse trigonometric functions, hyperbolic functions and inverse hyperbolic functions) can be brought to an exp-ln form. see e.g. Ambramowitz/Stegun 1970 or https://en.wikipedia.org/wiki/Hyperbolic_functions#Definitions – IV_ Jan 08 '24 at 20:22