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.) 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 entered mathematics through quadrature of the rectangular hyperbola xy = 1 by Gregoire de Saint Vincent in 1647, two millennia after Archimedes had produced The Quadrature of the Parabola. The area under the hyperbola was shown to have the scaling property of constant area for a constant ratio of bounds. The natural logarithm function so described was of limited service until 1748 when Leonhard Euler related it to functions where a constant is raised to a variable exponent, such as the exponential function where the constant base is e. By introducing these transcendental functions and noting the bijection property that implies an inverse function, some facility was provided for algebraic manipulations of the natural logarithm even if it is not an algebraic function.

Transcendental functions were first defined by 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 \$

${\displaystyle f_{2}(x)=c^{x}}$

$f_{3}(x)=x^{{x}}$

$f_4(x) = x^{\frac{1}{x}} \$

${\displaystyle f_{5}(x)=\log _{c}x}$

$f_6(x) = \sin{x}$

In particular, for ƒ₂ if we set c equal to e, the base of the natural logarithm, then we get that e^x is a transcendental function. Similarly, if we set c equal to e in ƒ₅, then we get that ${\displaystyle f_{5}(x)=\log _{e}x=\ln x}$ (that is, the natural logarithm) is a transcendental function.

Algebraic and transcendental functions

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) = e^z, 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 transcendental number 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:

• $\mathcal{E}(\exp)=\{0\}$,
• $\mathcal{E}(j)=\{\alpha\in\mathbf{H}\,:\,[\mathbf{Q}(\alpha): \mathbf{Q}]=2\}$,
  • Here j is Klein's j-invariant, H is the upper half-plane, and [Q(α): Q] is the degree of the number field Q(α). This result is due to Theodor Schneider.^[9]
• $\mathcal{E}(2^{x})=\mathbf{Q}$,
  • 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 α^β is transcendental. Thus the function 2^x could be replaced by c^x 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 consequence of Schanuel's conjecture in transcendental number theory would be that $\mathcal{E}(e^{e^x})=\emptyset.$
• 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.

See also

• Complex function
• Function (mathematics)
• Generalized function
• List of special functions and eponyms
• List of types of functions
• Rational function
• Special functions

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. cf. Abel–Ruffini theorem
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.

External links

• Definition of "Transcendental function" in the Encyclopedia of Math