Transcendental function

A transcendental function is a function that does not satisfy a polynomial equation whose coefficients are themselves roots of polynomials, in contrast to an algebraic function, which does satisfy such an equation. (The polynomials are sometimes required to have rational coefficients.) In other words, a transcendental function is a function that "transcends" algebra in the sense 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. This can be extended to functions of several variables.

Examples

The following functions are transcendental:

$f_{1}(x)=x^{\pi }\$

$f_{2}(x)=c^{x},\ c\neq 0,1$

$f_{3}(x)=x^{x}={{^{2}}x}\$

$f_{4}(x)=x^{\frac {1}{x}}\$

$f_{5}(x)=\log _{c}x,\ c\neq 0,1$

$f_{6}(x)=\sin {x}$

Note that 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 ln(x), the natural logarithm, is a transcendental function. For more information on the second notation of ƒ₃, see tetration.

Algebraic and transcendental functions

The logarithm and the exponential function are examples of transcendental functions. Transcendental function is a term often used to describe the trigonometric functions (sine, cosine, tangent, their reciprocals cotangent, secant, and cosecant, the now little-used versine, haversine, and coversine, their analogs the hyperbolic functions and so forth).

A function that is not transcendental is said to be algebraic. Examples of algebraic functions are rational functions and the square root function.

The operation of taking the indefinite integral of an algebraic function is a source of transcendental functions. For example, the logarithm function arose from the reciprocal function in an effort to find the area of a hyperbolic sector. Thus the hyperbolic angle and the hyperbolic functions sinh, cosh, and tanh are all transcendental.

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.

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.

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. 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, 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:

• ${\mathcal {E}}(\exp )=\{0\}$,

• ${\mathcal {E}}(j)=\{\alpha \in \mathbf {H} \,:\,[\mathbf {Q} (\alpha ):\mathbf {Q} ]=2\}$,
  • 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.

• ${\mathcal {E}}(2^{x})=\mathbf {Q}$,
  • 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 doesn't require one to assume this conjecture is the function 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. 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.

See also

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

External links

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