In mathematics, a transcendental function is an analytic function that does not satisfy a polynomial equation, in contrast to an algebraic function. In other words, a transcendental function "transcends" algebra in that it cannot be expressed algebraically.
The transcendental functions sine and cosine were tabulated from physical measurements in antiquity, as evidenced in Greece (Hipparchus) and India (jya and koti-jya). In describing Ptolemy's table of chords, an equivalent to a table of sines, Olaf Pedersen wrote:
The mathematical notion of continuity as an explicit concept is unknown to Ptolemy. That he, in fact, treats these functions as continuous appears from his unspoken presumption that it is possible to determine a value of the dependent variable corresponding to any value of the independent variable by the simple process of linear interpolation.
A revolutionary understanding of these circular functions occurred in the 17th century and was explicated by Leonhard Euler in 1748 in his Introduction to the Analysis of the Infinite. These ancient transcendental functions became known as continuous functions through quadrature of the rectangular hyperbola xy = 1 by Grégoire 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 hyperbolic 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.
The even and odd terms of this series provide sums denoting cosh(x) and sinh(x), so that . These transcendental hyperbolic functions can be converted into circular functions sine and cosine by introducing (−1)k into the series, resulting in alternating series. After Euler, mathematicians view the sine and cosine this way to relate the transcendence to logarithm and exponent functions, often through Euler's formula in complex number arithmetic.
The following functions are transcendental:
For the second function , if we set equal to , the base of the natural logarithm, then we get that is a transcendental function. Similarly, if we set equal to in , then we get that (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.
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 familiar transcendental functions, including the special functions of mathematical physics, are solutions of algebraic differential equations. Those that are not, such as the gamma and the zeta functions, are called transcendentally transcendental or hypertranscendental functions.
If is an algebraic function and is an algebraic number then is also an algebraic number. The converse is not true: there are entire transcendental functions such that is an algebraic number for any algebraic  For a given transcendental function the set of algebraic numbers giving algebraic results is called the exceptional set of that function. Formally it is defined by:
In many instances the exceptional set is fairly small. For example, this was proved by Lindemann in 1882. 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 if it can be calculated then it can often lead to results in transcendental number theory. Here are some other known exceptional sets:
- Klein's j-invariant where 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.
- Exponential function in base 2: This result is a corollary of the Gelfond–Schneider theorem, which states that if is algebraic, and is algebraic and irrational then is transcendental. Thus the function 2x could be replaced by cx for any algebraic c not equal to 0 or 1. Indeed, we have:
- A consequence of Schanuel's conjecture in transcendental number theory would be that .
- A function with empty exceptional set that does not require assuming Schanuel's conjecture is .
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 whose exceptional set is A. 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.
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 metres) is a nonsensical expression, unlike log(5 metres / 3 metres) or log(3) metres. One could attempt to apply a logarithmic identity to get log(5) + log(metres), which highlights the problem: applying a non-algebraic operation to a dimension creates meaningless results.
- Complex function
- Function (mathematics)
- Generalized function
- List of special functions and eponyms
- List of types of functions
- Rational function
- Special functions
- Townsend, E.J. (1915). Functions of a Complex Variable. H. Holt. p. 300. OCLC 608083625.
- Hazewinkel, Michiel (1993). Encyclopedia of Mathematics. Vol. 9. pp. 236.
- Waldschmidt, M. (2000). Diophantine approximation on linear algebraic groups. Springer. ISBN 978-3-662-11569-5.
- Pedersen, Olaf (1974). Survey of the Almagest. Odense University Press. p. 84. ISBN 87-7492-087-1.
- cf. Abel–Ruffini theorem
- Rubel, Lee A. (November 1989). "A Survey of Transcendentally Transcendental Functions". The American Mathematical Monthly. 96 (9): 777–788. doi:10.1080/00029890.1989.11972282. JSTOR 2324840.
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- Marques, D.; Lima, F.M.S. (2010). "Some transcendental functions that yield transcendental values for every algebraic entry". arXiv:1004.1668v1 [math.NT].
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- Wilkie, A.J. (1998). "An algebraically conservative, transcendental function". Paris VII Preprints. 66.