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A '''transcendental function''' is a [[function (mathematics)|function]] that does not satisfy a [[polynomial]] equation whose [[coefficient]]s are themselves polynomials, in contrast to an [[algebraic function]], which does satisfy such an equation.<ref>E. J. Townsend, ''Functions of a Complex Variable'', BiblioLife, LLC, (2009).</ref> (The polynomials are sometimes required to have [[rational number|rational]] coefficients.) In other words, a '''transcendental function''' is a function that "[[wiktionary:transcend|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 function]]s.
 
==Definition==
Formally, an [[analytic function]] &fnof;(''z'') of one real or complex variable ''z'' is transcendental if it is [[algebraic independence|algebraically independent]] of that variable.<ref>M. Waldschmidt, ''Diophantine approximation on linear algebraic groups'', Springer (2000).</ref> This can be extended to functions of several variables.
 
==Examples==
The following functions are transcendental:
 
:<math>f_1(x) = x^\pi \ </math>
:<math>f_2(x) = c^x, \ c \ne 0, 1</math>
:<math>f_3(x) = x^{x} = {{^2}x} \ </math>
:<math>f_4(x) = x^{\frac{1}{x}} \ </math>
:<math>f_5(x) = \log_c x, \ c \ne 0, 1</math>
:<math>f_6(x) = \sin{x}</math>
 
Note that in particular for ƒ<sub>2</sub> if we set c equal to ''e'', the [[exponential function|base of the natural logarithm]], then we get that ''e<sup>x</sup>'' is a transcendental function. Similarly, if we set ''c'' equal to ''e'' in ƒ<sub>5</sub>, then we get that ln(''x''), the [[natural logarithm]], is a transcendental function. For more information on the second notation of ƒ<sub>3</sub>, see [[tetration]].
 
==Algebraic and transcendental functions==
{{details|elementary function (differential algebra)}}
 
The [[logarithm]] and the [[exponential function]] are examples of transcendental functions.  ''Transcendental function'' is a term often used to describe the [[trigonometric function]]s ([[sine]], [[cosine]], [[tangent (trigonometric function)|tangent]], their reciprocals [[Trigonometric functions#Reciprocal functions|cotangent, secant, and cosecant]], the now little-used [[versine|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 [[Multiplicative inverse|reciprocal function]] in an effort to find the area of a [[hyperbolic sector]]. Thus the [[hyperbolic angle]] and the [[hyperbolic function]]s 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&nbsp;meters) is a nonsensical expression, unlike log(5&nbsp;meters&nbsp;/&nbsp;3&nbsp;meters)&nbsp; or &nbsp;log(3)&nbsp;meters. One could attempt to apply a [[logarithm]]ic identity to get log(10)&nbsp;+&nbsp;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 function|entire transcendental function]]s 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<sup>z</sup>'', 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,<ref>D. Marques, F. M. S. Lima, ''Some transcendental functions that yield transcendental values for every algebraic entry'', (2010) {{arxiv|1004.1668v1}}.</ref><ref>N. Archinard, ''Exceptional sets of hypergeometric series'', Journal of Number Theory '''101''' Issue 2 (2003), pp.244&ndash;269.</ref> that is the set
 
:<math>\mathcal{E}(f)=\{\alpha\in\overline{\mathbf{Q}}\,:\,f(\alpha)\in\overline{\mathbf{Q}}\}.</math>
 
If this set can be calculated then it can often lead to results in [[transcendence theory]]For example, [[Ferdinand von Lindemann|Lindemann]] proved in 1882 that the exceptional set of the exponential function is just {0}. In particular exp(1) = ''e'' is transcendentalAlso, since exp(''i''π) = -1 is algebraic we know that ''i''π 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:
 
*<math>\mathcal{E}(\exp)=\{0\}</math>,
 
*<math>\mathcal{E}(j)=\{\alpha\in\mathbf{H}\,:\,[\mathbf{Q}(\alpha): \mathbf{Q}]=2\}</math>,
**Here ''j'' is Klein's [[j-invariant|''j''-invariant]], '''H''' is the [[upper half-plane]], and ['''Q'''(α): '''Q'''] is the [[Degree of a field extension|degree]] of the [[Algebraic number field|number field]] '''Q'''(α).  This result is due to [[Theodor Schneider]].<ref>T. Schneider, ''Arithmetische Untersuchungen elliptischer Integrale'', Math. Annalen '''113''' (1937), pp.1&ndash;13.</ref>
 
*<math>\mathcal{E}(2^{x})=\mathbf{Q}</math>,
**This result is a corollary of the [[Gelfond&ndash;Schneider theorem]] which says that if α is algebraic and not 0 or 1, and if β is algebraic and irrational then α<sup>β</sup> is transcendental.  Thus the function 2<sup>''x''</sup> could be replaced by ''c<sup>x</sup>'' for any algebraic ''c'' not equal to 0 or 1. Indeed, we have:
 
*<math>\mathcal{E}(x^x)=\mathcal{E}(x^{\frac{1}{x}})=\mathbf{Q}\setminus\{0\}.</math>
 
*A consequence of [[Schanuel's conjecture]] in [[transcendence theory|transcendental number theory]] would be that <math>\mathcal{E}(e^{e^x})=\emptyset.</math>
 
*A function with empty exceptional set that doesn't require one to assume this conjecture is the function ƒ(''x'') = exp(1 + π''x'').
 
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''.<ref>M. Waldschmidt, ''Auxiliary functions in transcendental number theory'', The Ramanujan journal '''20''' no3, (2009), pp.341–373.</ref>  Since, as mentioned above, this includes taking ''A'' to be the whole set of algebraic numbers, there is no way to determine if a function is transcendental just by looking at its values at algebraic numbers.  In fact, [[Alex Wilkie]] showed that the situation is even worse: he constructed a transcendental function ƒ: '''R''' → '''R''' that is analytic everywhere but whose transcendence cannot be detected by any [[First-order logic|first-order]] method.<ref>A. Wilkie, ''An algebraically conservative, transcendental function'', Paris VII preprints, number 66, 1998.</ref>
 
==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]]
 
==References==
{{reflist}}
 
==External links==
*[http://www.encyclopediaofmath.org/index.php/Transcendental_function Definition of "Transcendental function" in the Encyclopedia of Math]
 
[[Category:Analytic functions]]
[[Category:Functions and mappings]]
[[Category:Meromorphic functions]]
[[Category:Special functions]]
[[Category:Types of functions]]

Revision as of 03:33, 7 March 2013

A transcendental function is a function that does not satisfy a polynomial equation whose coefficients are themselves polynomials, in contrast to an algebraic function, which does satisfy such an equation.[1] (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.[2] This can be extended to functions of several variables.

Examples

The following functions are transcendental:

Note that 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. For more information on the second notation of ƒ3, see tetration.

Algebraic and transcendental functions

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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) = 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,[3][4] that is the set

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:

  • 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.[6] Since, as mentioned above, this includes taking A to be the whole set of algebraic numbers, there is no way to determine if a function is transcendental just by looking at its values at algebraic numbers. In fact, Alex Wilkie showed that the situation is even worse: he constructed a transcendental function ƒ: RR that is analytic everywhere but whose transcendence cannot be detected by any first-order method.[7]

See also

References

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External links

  1. E. J. Townsend, Functions of a Complex Variable, BiblioLife, LLC, (2009).
  2. M. Waldschmidt, Diophantine approximation on linear algebraic groups, Springer (2000).
  3. D. Marques, F. M. S. Lima, Some transcendental functions that yield transcendental values for every algebraic entry, (2010) Template:Arxiv.
  4. N. Archinard, Exceptional sets of hypergeometric series, Journal of Number Theory 101 Issue 2 (2003), pp.244–269.
  5. T. Schneider, Arithmetische Untersuchungen elliptischer Integrale, Math. Annalen 113 (1937), pp.1–13.
  6. M. Waldschmidt, Auxiliary functions in transcendental number theory, The Ramanujan journal 20 no3, (2009), pp.341–373.
  7. A. Wilkie, An algebraically conservative, transcendental function, Paris VII preprints, number 66, 1998.