Closed-Form – How to Show $A(z,e^z)$ and $A(\ln (z),z)$ Have No Elementary Inverse

closed-formelementary-functionsinverse function

I'm generally interested in equation solving in closed form.

The algebraic functions are meant with complex coefficients.

Let
$A$ an algebraic function of two complex variables,
$F$ with $F(z)=A(z,e^z)$ a bijective function of one complex variable with a complex domain that doesn't contain isolated points.

My question is:
How can we show that the inverse of the function $F$ is not an elementary function?

If we have shown that $F$ has no elementary inverse, the analog statement can be derived for a function $F_1$ with $F_1(z)=A(\ln(z),z)$ because $F_1(z)=F(\ln(z))$.

A classic example is the function $F$ with $F(z)=ze^z$. Its inverse relation is Lambert W. There are various proofs that show that the branches of Lambert W aren't elementary, e.g. [Bronstein/Corless/Davenport/Jeffrey 2008].

I want to point out the method of J. Liouville and J. F. Ritt in my answer and mention the sub-problem when $A$ is a polynomial function.

Further answers and methods are welcome.

Best Answer

Consider that the existence of elementary inverses and the solvability of equations by elementary numbers are two different mathematical problems, as [Chow 1999] points out.

We have $F(z)=A(z,e^z)$. Let's denote the inverse of $F$ by $y$. For the inverse $y$, $F(y(x))=x$ applies. So we get

$$A(y(x),e^{y(x)})=x\tag{1}$$

with $x$ a continuous variable.
Because $y$ is the inverse of $F$, both $F$ and $y$ are injective, and $y$ cannot be a constant function therefore.

1.)

There is a proof in [Liouville 1938] p. 536 - 539 for the equation $$\log y=F(x,y),$$ where $F$ is an algebraic equation involving both $x$ and $y$. Liouville proves that $y$ is not an elementary function of positive order if $F'_x(x,y),F'_y(x,y)\neq 0$.
(Someone should translate Liouville's works into English.)

2.)

I found that the proof from [Ritt 1948] p. 59 - 62 for Kepler's equation $z-c\ \sin(z)=x\ (c\in\mathbb{C})$ can also be used for the equation $A(z,e^z)=x$. Ritt presents the definition and method of elementary functions of order $n$ of Liouville ([Liouville 1837], [Liouville 1838]).

Ritt's proof goes for the equation $$z=\log w(x,z)\tag{11}$$ "where $w$, algebraic in $x$ and $z$, involves $z$ effectively. Equation (11) is certainly not satisfied by a nonconstant algebraic $z(x)$."
Ritt then proves that (11) has no solution $z$ that is an elementary function of order $n>0$.

The main theorem in [Ritt 1925], that is also proved in [Risch 1979], implies that our function term $F(z)=A(z,e^z)$ is not in a form that allows to decide if the function $F$ has an elementary inverse or not.
But with the above proofs from Liouville and Ritt, we are able to prove the non-existence of an elementary inverse of $F$.

3.)

For functions $A$ for which equation (1) leads to an equation $P(z,e^z)=0\ $ (2), where $P$ is an irreducible polynomial function of two complex variables with algebraic coefficients, the main theorem in [Lin 1983] states that equation (2) doesn't have nonzero solutions $z$ that are elementary numbers.

It follows with the theorem in Proof Check: Non-existence of the inverse function in a given class of functions, that the function $F$ with $F(z)=P(z,e^z)$ doesn't have an inverse that's an elementary functions.

4.)

Possibly the method of [Rosenlicht 1969] /[Rosenlicht 1976] could also be used to prove the problem from the question.
$\ $

[Bronstein/Corless/Davenport/Jeffrey 2008] Bronstein, M.; Corless, R. M.; Davenport, J. H., Jeffrey, D. J.: Algebraic properties of the Lambert W Function from a result of Rosenlicht and of Liouville. Integral Transforms and Special Functions 19 (2008) (10) 709-712

[Chow 1999] Chow, T.: What is a closed-form number. Am. Math. Monthly 106 (1999) (5) 440-448

[Lin 1983] Ferng-Ching Lin: Schanuel's Conjecture Implies Ritt's Conjectures. Chin. J. Math. 11 (1983) (1) 41-50

[Liouville 1837] Liouville, J.: Mémoire sur la classification des transcendantes et sur l'impossibilité d'exprimer les racines de certaines équations en fonction finie explicite des coefficients. J. de mathématiques pures et appliquées 2 (1837) 56-105

[Liouville 1838] Liouville, J.: Suite du mémoire sur la classification des transcendantes, et sur l'impossibilité d'exprimer les racines de certaines équations en fonction finie explicite des coefficients. J. de Mathématiques Pures et Appliquées 3 (1838) 523-547

[Risch 1979] Risch, R. H.: Algebraic Properties of the Elementary Functions of Analysis. Amer. J. Math. 101 (1979) (4) 743-759

[Ritt 1925] Ritt, J. F.: Elementary functions and their inverses. Trans. Amer. Math. Soc. 27 (1925) (1) 68-90

[Ritt 1948] Ritt, J. F.: Integration in finite terms. Liouville's theory of elementary methods. Columbia University Press, New York, 1948

[Rosenlicht 1969] Rosenlicht, M.: On the explicit solvability of certain transcendental equations. Publications mathématiques de l'IHÉS 36 (1969) 15-22

[Rosenlicht 1976] Rosenlicht, M.: On Liouville's theory of elementary functions. Pacific J. Math. 65 (1976) (2) 485-492

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