Series expansion explained

In mathematics, a series expansion is a technique that expresses a function as an infinite sum, or series, of simpler functions. It is a method for calculating a function that cannot be expressed by just elementary operators (addition, subtraction, multiplication and division).^[1]

The resulting so-called series often can be limited to a finite number of terms, thus yielding an approximation of the function. The fewer terms of the sequence are used, the simpler this approximation will be. Often, the resulting inaccuracy (i.e., the partial sum of the omitted terms) can be described by an equation involving Big O notation (see also asymptotic expansion). The series expansion on an open interval will also be an approximation for non-analytic functions.^[2]

Types of series expansions

There are several kinds of series expansions, listed below.

Taylor series

A Taylor series is a power series based on a function's derivatives at a single point.^[3] More specifically, if a function

f: U\to\R is infinitely differentiable around a point x _{0} , then the Taylor series of f around this point is given by\sum ^{inf ty} _{n =0} (f ^{(n)} (x _{0)})/(n!) (x - x ^{n} _{0 )}

under the convention

0 ^{0} : = 1 .^[4] The Maclaurin series of f is its Taylor series about x _{0} = 0 .^[5]

Laurent series

A Laurent series is a generalization of the Taylor series, allowing terms with negative exponents; it takes the form $\backslash sum\_^\; c\_k\; (z\; -\; a)^k$ and converges in an annulus.^[6] In particular, a Laurent series can be used to examine the behavior of a complex function near a singularity by considering the series expansion on an annulus centered at the singularity.

Dirichlet series

A general Dirichlet series is a series of the form $\backslash sum\_^\; a\_ne^.$ One important special case of this is the ordinary Dirichlet series $\backslash sum\_^\backslash frac.$^[7] Used in number theory.

Fourier series

A Fourier series is an expansion of periodic functions as a sum of many sine and cosine functions.^[8] More specifically, the Fourier series of a function

f(t) of period 2 L is given by the expression$$a\_0\; +\; \backslash sum\_^\; \backslash left[a\_n\backslash cos\backslash left(\backslash frac\{n\backslash pi\; t\}\{L\}\backslash right)\; +\; b\_n\backslash sin\backslash left(\backslash frac\{n\backslash pi\; t\}\{L\}\backslash right)\backslash right]$$where the coefficients are given by the formulae^[9]

Other series

• In acoustics, e.g., the fundamental tone and the overtones together form an example of a Fourier series.
• Newtonian series
• Legendre polynomials

Used in physics to describe an arbitrary electrical field as a superposition of a dipole field, a quadrupole field, an octupole field, etc.

• Zernike polynomials

Used in optics to calculate aberrations of optical systems. Each term in the series describes a particular type of aberration.

• The Stirling series$$\backslash text\backslash Gamma\backslash left(z\backslash right)\backslash sim\backslash left(z-\backslash tfrac\backslash right)\backslash ln\; z-z+\backslash tfrac\backslash ln\backslash left(2\backslash pi\backslash right)+\backslash sum\_^\backslash frac$$is an approximation of the log-gamma function.^[10]

Examples

The following is the Taylor series of

e ^{x} :$$e^x=\backslash sum^\_\backslash frac=\; 1\; +\; x\; +\; \backslash frac\; +\; \backslash frac...$$^{[11] [12]}

The Dirichlet series of the Riemann zeta function is$$\backslash zeta(s)\; :=\; \backslash sum\_^\; \backslash frac\; =\; \backslash frac\; +\; \backslash frac\; +\; \backslash cdots$$

References

1. Web site: 2013-11-07. Series and Expansions. 2021-12-24. Mathematics LibreTexts. en.
2. Book: Gil. Amparo. Numerical Methods for Special Functions. Segura. Javier. Temme. Nico M.. 2007-01-01. SIAM. 978-0-89871-782-2. en.
3. Web site: 27 December 2013 . Taylor series - Encyclopedia of Mathematics . 22 March 2022 . encyclopediaofmath.org.
4. Book: Edwards . C. Henry . Elementary Differential Equations with Boundary Value Problems . Penney . David E. . 2008 . 978-0-13-600613-8 . 196 . Pearson/Prentice Hall . en.
5. Web site: Weisstein . Eric W. . Maclaurin Series . 2022-03-22 . mathworld.wolfram.com . en.
6. Web site: Laurent series - Encyclopedia of Mathematics . 2022-03-22 . encyclopediaofmath.org.
7. Web site: 26 January 2022 . Dirichlet series - Encyclopedia of Mathematics . 22 March 2022 . encyclopediaofmath.org.
8. Web site: Fourier series - Encyclopedia of Mathematics . 2022-03-22 . encyclopediaofmath.org.
9. Book: Edwards . C. Henry . Elementary Differential Equations with Boundary Value Problems . Penney . David E. . 2008 . 978-0-13-600613-8 . 558, 564 . Pearson/Prentice Hall . en.
10. Web site: DLMF: 5.11 Asymptotic Expansions . 22 March 2022 . dlmf.nist.gov.
11. Web site: Weisstein. Eric W.. Exponential Function. 2021-08-12. mathworld.wolfram.com. en.
12. Web site: 5 June 2020. Exponential function - Encyclopedia of Mathematics. 12 August 2021. encyclopediaofmath.org.