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error function: Definition from Answers.com

Plot of the error function

In mathematics, the error function (also called the Gauss error function) is a non-elementary function which occurs in probability, statistics and partial differential equations. It is defined as:

$\operatorname{erf}(x) = \frac{2}{\sqrt{\pi}}\int_0^x e^{-t^2} dt.$

Properties

The error function is odd:

$\operatorname{erf} (-x) = -\operatorname{erf} (x).$

Also, for any complex number x one has

$\operatorname{erf} (x^{*}) = \operatorname{erf}(x)^{*}$

where x * is the complex conjugate of x.

The integral cannot be evaluated in closed form in terms of elementary functions, but by expanding the integrand in a Taylor series, one obtains the Taylor series for the error function as follows:

$\operatorname{erf}(x)= \frac{2}{\sqrt{\pi}}\sum_{n=0}^\infin\frac{(-1)^n x^{2n+1}}{n! (2n+1)} =\frac{2}{\sqrt{\pi}} \left(x-\frac{x^3}{3}+\frac{x^5}{10}-\frac{x^7}{42}+\frac{x^9}{216}-\ \cdots\right)$

which holds for every real number x, and also throughout the complex plane.

(This result arises from the Taylor series expansion of $e^{-x^2}$ , which is $\sum_{n=0}\frac{(-1)^n x^{2n}}{n!}$ , which we then integrate term by term.)

The error function at infinity is exactly 1 (see Gaussian integral).

The derivative of the error function follows immediately from its definition:

$\frac{d}{dx}\,\mathrm{erf}(x)=\frac{2}{\sqrt{\pi}}\,e^{-x^2}.$

The inverse error function has series

$\operatorname{erf}^{-1}(x)=\sum_{k=0}^\infin\frac{c_k}{2k+1}\left (\frac{\sqrt{\pi}}{2}x\right )^{2k+1}, \,\!$

where c₀ = 1 and

$c_k=\sum_{m=0}^{k-1}\frac{c_m c_{k-1-m}}{(m+1)(2m+1)} = \left\{1,1,\frac{7}{6},\frac{127}{90},\ldots\right\}.$

So we have the series expansion

$\operatorname{erf}^{-1}(x)=\frac{1}{2}\sqrt{\pi}\left (x+\frac{\pi x^3}{12}+\frac{7\pi^2 x^5}{480}+\frac{127\pi^3 x^7}{40320}+\frac{4369\pi^4 x^9}{5806080}+\frac{34807\pi^5 x^{11}}{182476800}+\cdots\right ). \,\!$ [1]

Plot of the complementary error function

The complementary error function, denoted erfc, is defined in terms of the error function:

$\mbox{erfc}(x) = 1-\mbox{erf}(x) = \frac{2}{\sqrt{\pi}} \int_x^{\infty} e^{-t^2}\,dt.$

The complex error function, denoted w(x), (also known as the Faddeeva function) is also defined in terms of the error function:

$w(x) = e^{-x^2}{\textrm{erfc}}(-ix).\,\!$

Applications

When the results of a series of measurements are described by a normal distribution with standard deviation σ and expected value 0, then $\operatorname{erf}\,\left(\,\frac{a}{\sigma \sqrt{2}}\,\right)$ is the probability that the error of a single measurement lies between −a and +a.

The error and complementary error functions occur, for example, in solutions of the heat equation when boundary conditions are given by the Heaviside step function.

In digital optical communication system, BER is expressed by:

$\mathrm{BER} = 0.5\,\operatorname{erfc}\left( \frac{\mu_1 - \mu_2}{\sqrt{2}\left(\sigma_1 + \sigma_2\right)} \right).$

Asymptotic expansion

A useful asymptotic expansion of the complementary error function (and therefore also of the error function) for large x is

$\mathrm{erfc}(x) = \frac{e^{-x^2}}{x\sqrt{\pi}}\left [1+\sum_{n=1}^\infty (-1)^n \frac{1\cdot3\cdot5\cdots(2n-1)}{(2x^2)^n}\right ]=\frac{e^{-x^2}}{x\sqrt{\pi}}\sum_{n=0}^\infty (-1)^n \frac{(2n)!}{n!(2x)^{2n}}.\,$

This series diverges for every finite x. However, in practice only the first few terms of this expansion are needed to obtain a good approximation of erfc(x), whereas the Taylor series given above converges very slowly.

Another approximation is given by

$(\operatorname{erf}(x))^2\approx 1-\exp\left(-x^2\frac{4/\pi+ax^2}{1+ax^2}\right)$

where

$a = \frac{-8}{3\pi}\frac{\pi-3}{\pi-4}.$

Related functions

The error function is essentially identical to the standard normal cumulative distribution function, denoted Φ, as they differ only by scaling and translation. Indeed,

$\Phi(x) = \frac{1}{2}\left[1+\mbox{erf}\left(\frac{x}{\sqrt{2}}\right)\right]\,.$

The inverse of $\Phi\,$ is known as the normal quantile function, or probit function and may be expressed in terms of the inverse error function as

$\operatorname{probit}(p) = \Phi^{-1}(p) = \sqrt{2}\,\operatorname{erf}^{-1}(2p-1).$

The standard normal cdf is used more often in probability and statistics, and the error function is used more often in other branches of mathematics.

The error function is a special case of the Mittag-Leffler function, and can also be expressed as a confluent hypergeometric function (Kummer's function):

$\mathrm{erf}(x)= \frac{2x}{\sqrt{\pi}}\,_1F_1\left(\frac{1}{2},\frac{3}{2},-x^2\right).$

It has a simple expression in terms of the Fresnel integral. In terms of the Regularized Gamma function P and the incomplete gamma function,

$\operatorname{erf}(x)=\operatorname{sgn}(x) P\left(\frac{1}{2}, x^2\right)={\operatorname{sgn}(x) \over \sqrt{\pi}}\gamma\left(\frac{1}{2}, x^2\right).$

is the sign function.

Generalized error functions

Some authors discuss the more general functions

$E_n(x) = \frac{n!}{\sqrt{\pi}} \int_0^x e^{-t^n}\,dt =\frac{n!}{\sqrt{\pi}}\sum_{p=0}^\infin(-1)^p\frac{x^{np+1}}{(np+1)p!}\,.$

E₂(x) is the error function.

Graph of generalized error functions E_n(x). Grey curve: E₁(x) = 1 − e^−x, red curve: erf(x) = E₂(x), green curve: E₃(x), blue curve: E₄(x), and yellow curve: E₅(x). (The yellow curve is quite close to the y-axis and may not be visible.) After division by n!, all the E_n for odd n look similar (but not identical) to each other. Similarly, the E_n for even n look similar (but not identical) to each other after a simple division by n!. The E_n with odd and even n look similar on the positive x side of the graph.

Iterated integrals of the complementary error function

The iterated integrals of the complementary error function are defined by

$\mathrm i^n \operatorname{erfc}\, z = \int_z^\infty \mathrm i^{n-1} \operatorname{erfc}\, \zeta\;\mathrm d \zeta.\,$

They have the power series

$\mathrm i^n \operatorname{erfc}\, z = \sum_{j=0}^\infty \frac{(-z)^j}{2^{n-j}j! \Gamma \left( 1 + \frac{n-j}{2}\right)}\,,$

from which follow the symmetry properties

$\mathrm i^{2m} \operatorname{erfc} (-z) = - \mathrm i^{2m} \operatorname{erfc}\, z + \sum_{q=0}^m \frac{z^{2q}}{2^{2(m-q)-1}(2q)! (m-q)!}$

and

$\mathrm i^{2m+1} \operatorname{erfc} (-z) = \mathrm i^{2m+1} \operatorname{erfc}\, z + \sum_{q=0}^m \frac{z^{2q+1}}{2^{2(m-q)-1}(2q+1)! (m-q)!}\,.$

Implementation

C/C++: It is implemented as the function erf() and erfc() in the header math.h or cmath in the GNU version. This is not part of standard so please check the compiler documentation before use.

References

Milton Abramowitz and Irene A. Stegun, eds. Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables. New York: Dover, 1972. (See Chapter 7)

External links

MathWorld - Erf

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