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). 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.

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. 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 under the convention 0^0 := 1. The Maclaurin series of f is its Taylor series about x_0 = 0.

Laurent series

A Laurent series is a generalization of the Taylor series, allowing terms with negative exponents; it takes the form and converges in an annulus. 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 One important special case of this is the ordinary Dirichlet series Used in number theory.

Fourier series

A Fourier series is an expansion of periodic functions as a sum of many sine and cosine functions. More specifically, the Fourier series of a function f(x) of period 2L is given by the expressionwhere the coefficients are given by the formulae

Other series

Examples

The following is the Taylor series of e^x: The Dirichlet series of the Riemann zeta function is