Elementary function

In mathematics, an elementary function is a function of a single variable (typically real or complex) that is defined as taking sums, products, roots and compositions of finitely many polynomial, rational, trigonometric, hyperbolic, and exponential functions, and their inverses (e.g., arcsin, log, or x^1/n).^[1]

All elementary functions are continuous on their domains.

Elementary functions were introduced by Joseph Liouville in a series of papers from 1833 to 1841.^[2]^[3]^[4] An algebraic treatment of elementary functions was started by Joseph Fels Ritt in the 1930s.^[5] Many textbooks and dictionaries do not give a precise definition of the elementary functions, and mathematicians differ on it.^[6]

Examples

Basic examples

Elementary functions of a single variable $x$ include:

Constant functions: $2,\ \pi ,\ e,$ etc.
Rational powers of $x$ : $x,\ x^{2},\ {\sqrt {x}}\ (x^{\frac {1}{2}}),\ x^{\frac {2}{3}},$ etc.
Exponential functions: $e^{x},\ a^{x}$
Logarithms: $\log x,\ \log _{a}x$
Trigonometric functions: $\sin x,\ \cos x,\ \tan x,$ etc.
Inverse trigonometric functions: $\arcsin x,\ \arccos x,$ etc.
Hyperbolic functions: $\sinh x,\ \cosh x,$ etc.
Inverse hyperbolic functions: $\operatorname {arsinh} x,\ \operatorname {arcosh} x,$ etc.
All functions obtained by adding, subtracting, multiplying or dividing a finite number of any of the previous functions^[7]
All functions obtained by root extraction of a polynomial with coefficients in elementary functions^[8]
All functions obtained by composing a finite number of any of the previously listed functions

Certain elementary functions of a single complex variable $z$ , such as ${\sqrt {z}}$ and $\log z$ , may be multivalued. Additionally, certain classes of functions may be obtained by others using the final two rules. For example, the exponential function $e^{z}$ composed with addition, subtraction, and division provides the hyperbolic functions, while initial composition with $iz$ instead provides the trigonometric functions.

Composite examples

Examples of elementary functions include:

Addition, e.g. ( $x$ +1)
Multiplication, e.g. (2 $x$ )
Polynomial functions
${\frac {e^{\tan x}}{1+x^{2}}}\sin \left({\sqrt {1+(\log x)^{2}}}\right)$
$-i\log \left(x+i{\sqrt {1-x^{2}}}\right)$

The last function is equal to $\arccos x$ , the inverse cosine, in the entire complex plane.

All monomials, polynomials, rational functions and algebraic functions are elementary.

The absolute value function, for real $x$ , is also elementary as it can be expressed as the composition of a power and root of $x$ : ${\textstyle |x|={\sqrt {x^{2}}}}$ .^{[dubious – discuss]}

Non-elementary functions

Many mathematicians exclude non-analytic functions such as the absolute value function or discontinuous functions such as the step function,^[9]^[6] but others allow them. Some have proposed extending the set to include, for example, the Lambert W function.^[10]

Some examples of functions that are not elementary:

tetration
the gamma function
non-elementary Liouvillian functions, including
- the exponential integral (Ei), logarithmic integral (Li or li) and Fresnel integrals (S and C).
- the error function, $\mathrm {erf} (x)={\frac {2}{\sqrt {\pi }}}\int _{0}^{x}e^{-t^{2}}\,dt,$ a fact that may not be immediately obvious,^{[further explanation needed]} but can be proven using the Risch algorithm.
other nonelementary integrals, including the Dirichlet integral and elliptic integral.

Closure

It follows directly from the definition that the set of elementary functions is closed under arithmetic operations, root extraction and composition. The elementary functions are closed under differentiation. They are not closed under limits and infinite sums. Importantly, the elementary functions are not closed under integration, as shown by Liouville's theorem, see nonelementary integral. The Liouvillian functions are defined as the elementary functions and, recursively, the integrals of the Liouvillian functions.

Differential algebra

The mathematical definition of an elementary function, or a function in elementary form, is considered in the context of differential algebra. A differential algebra is an algebra with the extra operation of derivation (algebraic version of differentiation). Using the derivation operation new equations can be written and their solutions used in extensions of the algebra. By starting with the field of rational functions, two special types of transcendental extensions (the logarithm and the exponential) can be added to the field building a tower containing elementary functions.

A differential field F is a field F₀ (rational functions over the rationals Q for example) together with a derivation map u → ∂u. (Here ∂u is a new function. Sometimes the notation u′ is used.) The derivation captures the properties of differentiation, so that for any two elements of the base field, the derivation is linear

\partial (u+v)=\partial u+\partial v

and satisfies the Leibniz product rule

\partial (u\cdot v)=\partial u\cdot v+u\cdot \partial v\,.

An element h is a constant if ∂h = 0. If the base field is over the rationals, care must be taken when extending the field to add the needed transcendental constants.

A function u of a differential extension F[u] of a differential field F is an elementary function over F if the function u

is algebraic over F, or
is an exponential, that is, ∂u = u ∂a for a ∈ F, or
is a logarithm, that is, ∂u = ∂a / a for a ∈ F.

(see also Liouville's theorem)

Notes

^ Spivak, Michael. (1994). Calculus (3rd ed.). Houston, Tex.: Publish or Perish. p. 359. ISBN 0914098896. OCLC 31441929.
^ Liouville 1833a.
^ Liouville 1833b.
^ Liouville 1833c.
^ Ritt 1950.
^ ^a ^b Subbotin, Igor Ya.; Bilotskii, N. N. (March 2008). "Algorithms and Fundamental Concepts of Calculus" (PDF). Journal of Research in Innovative Teaching. 1 (1): 82–94.
^ Ordinary Differential Equations. Dover. 1985. p. 17. ISBN 0-486-64940-7.
^ Weisstein, Eric W. "Elementary Function." From MathWorld
^ Risch, Robert H. (1979). "Algebraic Properties of the Elementary Functions of Analysis". American Journal of Mathematics. 101 (4): 743–759. doi:10.2307/2373917. ISSN 0002-9327. JSTOR 2373917.
^ Stewart, Seán (2005). "A new elementary function for our curricula?" (PDF). Australian Senior Mathematics Journal. 19 (2): 8–26.

References

Liouville, Joseph (1833a). "Premier mémoire sur la détermination des intégrales dont la valeur est algébrique". Journal de l'École Polytechnique. tome XIV: 124–148.
Liouville, Joseph (1833b). "Second mémoire sur la détermination des intégrales dont la valeur est algébrique". Journal de l'École Polytechnique. tome XIV: 149–193.
Liouville, Joseph (1833c). "Note sur la détermination des intégrales dont la valeur est algébrique". Journal für die reine und angewandte Mathematik. 10: 347–359.
Ritt, Joseph (1950). Differential Algebra. AMS.
Rosenlicht, Maxwell (1972). "Integration in finite terms". American Mathematical Monthly. 79 (9): 963–972. doi:10.2307/2318066. JSTOR 2318066.

External links

[1] Spivak, Michael. (1994). Calculus (3rd ed.). Houston, Tex.: Publish or Perish. p. 359. ISBN 0914098896. OCLC 31441929.

[2] Liouville 1833a.

[3] Liouville 1833b.

[4] Liouville 1833c.

[5] Ritt 1950.

[:0-6] Subbotin, Igor Ya.; Bilotskii, N. N. (March 2008). "Algorithms and Fundamental Concepts of Calculus" (PDF). Journal of Research in Innovative Teaching. 1 (1): 82–94.

[7] Ordinary Differential Equations. Dover. 1985. p. 17. ISBN 0-486-64940-7.

[8] Weisstein, Eric W. "Elementary Function." From MathWorld

[9] Risch, Robert H. (1979). "Algebraic Properties of the Elementary Functions of Analysis". American Journal of Mathematics. 101 (4): 743–759. doi:10.2307/2373917. ISSN 0002-9327. JSTOR 2373917.

[10] Stewart, Seán (2005). "A new elementary function for our curricula?" (PDF). Australian Senior Mathematics Journal. 19 (2): 8–26.

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