In mathematics, a series expansion is a method for calculating a function that cannot be expressed by just elementary operators (addition, subtraction, multiplication and division).

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.

There are several kinds of series expansions, such as:

• Taylor series: A power series based on a function’s derivatives at a single point.
• Maclaurin series: A special case of a Taylor series, centred at zero.
• Laurent series: An extension of the Taylor series, allowing negative exponent values.
• Dirichlet series: Used in number theory.
• Fourier series: Describes periodical functions as a series of sine and cosine functions. 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.
• Stirling series: Used as an approximation for factorials.

For more details, refer to the articles mentioned.

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