# Transcendental function

A transcendental function is an analytic function that does not satisfy a polynomial equation, in contrast to an algebraic function.[1][2] (The polynomials are sometimes required to have rational coefficients.[citation needed]) In other words, a transcendental function "transcends" algebra in that it cannot be expressed in terms of a finite sequence of the algebraic operations of addition, multiplication, and root extraction.

Examples of transcendental functions include the exponential function, the logarithm, and the trigonometric functions.

## Definition

Formally, an analytic function ƒ(z) of one real or complex variable z is transcendental if it is algebraically independent of that variable.[3] This can be extended to functions of several variables.

## History

Transcendental functions were first defined by Leonhard Euler in his Introductio (1748) as functions either not definable by the "ordinary operations of algebra", or defined by such operations "repeated infinitely often". But this definition is unsatisfactory, since some functions defined with infinitely many operations remain algebraic or even rational. The theory was further developed by Gotthold Eisenstein (Eisenstein's theorem), Eduard Heine, and others.[4]

## Examples

The following functions are transcendental:

$f_1(x) = x^\pi \$
$f_2(x) = c^x, \ c \ne 0, 1$
$f_3(x) = x^{x}$
$f_4(x) = x^{\frac{1}{x}} \$
$f_5(x) = \log_c x, \ c \ne 0, 1$
$f_6(x) = \sin{x}$

In particular, for ƒ2 if we set c equal to e, the base of the natural logarithm, then we get that ex is a transcendental function. Similarly, if we set c equal to e in ƒ5, then we get that ln(x), the natural logarithm, is a transcendental function.

## Algebraic and transcendental functions

For more details on this topic, see Elementary function (differential algebra).

The most familiar transcendental functions are the logarithm, the exponential (with any non-trivial base), the trigonometric, and the hyperbolic functions, and the inverses of all of these. Less familiar are the special functions of analysis, such as the gamma, elliptic, and zeta functions, all of which are transcendental. The generalized hypergeometric and Bessel functions are transcendental in general, but algebraic for some special parameter values.

A function that is not transcendental is algebraic. Simple examples of algebraic functions are the rational functions and the square root function, but in general, algebraic functions cannot be defined as finite formulas of the elementary functions.[5]

The indefinite integral of many algebraic functions is transcendental. For example, the logarithm function arose from the reciprocal function in an effort to find the area of a hyperbolic sector.

Differential algebra examines how integration frequently creates functions that are algebraically independent of some class, such as when one takes polynomials with trigonometric functions as variables.

## Transcendentally transcendental functions

Most of the familiar transcendental functions, including the special functions of mathematical physics, are solutions of algebraic differential equations. Those which are not, such as the gamma and the zeta functions, are called transcendentally transcendental or hypertranscendental functions.

## Exceptional set

If ƒ(z) is an algebraic function and α is an algebraic number then ƒ(α) will also be an algebraic number. The converse is not true: there are entire transcendental functions ƒ(z) such that ƒ(α) is an algebraic number for any algebraic α.[6] In many instances, however, the set of algebraic numbers α where ƒ(α) is algebraic is fairly small. For example, if ƒ is the exponential function, ƒ(z) = ez, then the only algebraic number α where ƒ(α) is also algebraic is α = 0, where ƒ(α) = 1. For a given transcendental function this set of algebraic numbers giving algebraic results is called the exceptional set of the function,[7][8] that is the set

$\mathcal{E}(f)=\{\alpha\in\overline{\mathbf{Q}}\,:\,f(\alpha)\in\overline{\mathbf{Q}}\}.$

If this set can be calculated then it can often lead to results in transcendence theory. For example, Lindemann proved in 1882 that the exceptional set of the exponential function is just {0}. In particular exp(1) = e is transcendental. Also, since exp(iπ) = -1 is algebraic we know that iπ cannot be algebraic. Since i is algebraic this implies that π is a transcendental number.

In general, finding the exceptional set of a function is a difficult problem, but it has been calculated for some functions:

• $\mathcal{E}(\exp)=\{0\}$,
• $\mathcal{E}(2^{x})=\mathbf{Q}$,
• This result is a corollary of the Gelfond–Schneider theorem which says that if α is algebraic and not 0 or 1, and if β is algebraic and irrational then αβ is transcendental. Thus the function 2x could be replaced by cx for any algebraic c not equal to 0 or 1. Indeed, we have:
• $\mathcal{E}(x^x)=\mathcal{E}(x^{\frac{1}{x}})=\mathbf{Q}\setminus\{0\}.$
• A function with empty exceptional set that does not require assuming Schanuel's conjecture is ƒ(x) = exp(1 + πx).

While calculating the exceptional set for a given function is not easy, it is known that given any subset of the algebraic numbers, say A, there is a transcendental function ƒ whose exceptional set is A.[10] The subset does not need to be proper, meaning that A can be the set of algebraic numbers. This directly implies that there exist transcendental functions that produce transcendental numbers only when given transcendental numbers. Alex Wilkie also proved that there exist transcendental functions for which first-order-logic proofs about their transcendence do not exist by providing an exemplary analytic function.[11]

## Dimensional analysis

In dimensional analysis, transcendental functions are notable because they make sense only when their argument is dimensionless (possibly after algebraic reduction). Because of this, transcendental functions can be an easy-to-spot source of dimensional errors. For example, log(5 meters) is a nonsensical expression, unlike log(5 meters / 3 meters)  or  log(3) meters. One could attempt to apply a logarithmic identity to get log(10) + log(m), which highlights the problem: applying a non-algebraic operation to a dimension creates meaningless results.

## References

1. ^ E. J. Townsend, Functions of a Complex Variable, 1915, p. 300
2. ^ Michiel Hazewinkel, Encyclopedia of Mathematics, 1993, 9:236
3. ^ M. Waldschmidt, Diophantine approximation on linear algebraic groups, Springer (2000).
4. ^ Amy Dahan-Dalmédico, Jeanne Peiffer, History of Mathematics: Highways and Byways, 2010, p. 240
5. ^
6. ^ A. J. van der Poorten. 'Transcendental entire functions mapping every algebraic number field into itself’, J. Austral. Math. Soc. 8 (1968), 192–198
7. ^ D. Marques, F. M. S. Lima, Some transcendental functions that yield transcendental values for every algebraic entry, (2010) arXiv:1004.1668v1.
8. ^ N. Archinard, Exceptional sets of hypergeometric series, Journal of Number Theory 101 Issue 2 (2003), pp.244–269.
9. ^ T. Schneider, Arithmetische Untersuchungen elliptischer Integrale, Math. Annalen 113 (1937), pp.1–13.
10. ^ M. Waldschmidt, Auxiliary functions in transcendental number theory, The Ramanujan Journal 20 no3, (2009), pp.341–373.
11. ^ A. Wilkie, An algebraically conservative, transcendental function, Paris VII preprints, number 66, 1998.