The Faddeeva function or Kramp function is a scaled complex complementary error function,
Contents
It is related to the Fresnel integral, to Dawson's integral, and to the Voigt function.
The function arises in various physical problems in describing electromagnetic response in complicated media.
Real and imaginary parts
The decomposition into real and imaginary parts is usually written
where V and L are called the real and imaginary Voigt functions, since V(x,y) is the Voigt profile (up to prefactors).
Sign inversion
For sign-inverted arguments, the following both apply:
and
where * denotes complex conjugate.
Integral representation
The Faddeeva function occurs as
meaning that it is a convolution of a Gaussian with a simple pole.
History
The function was tabulated by Faddeeva and Terent'ev in 1954. It appears as nameless function w(z) in Abramowitz and Stegun (1964), formula 7.1.3. The name Faddeeva function was apparently introduced by Poppe and Wijers in 1990; previously, it was known as Kramp's function (probably after Christian Kramp).
Early implementations used methods by Gautschi (1969/70; ACM Algorithm 363) or by Humlicek (1982). A more efficient algorithm was proposed by Poppe and Wijers (1990; ACM Algorithm 680). Weideman (1994) proposed a particularly short algorithm that takes no more than eight lines of Matlab code. Zaghloul and Ali pointed out deficiencies of previous algorithms and proposed a new one (2011; ACM Algorithm 916). Another algorithm has been proposed by Abrarov and Quine (2011/2012).
Implementations
Two software implementations, which are free for non-commercial use only (and hence are not Free and open-source software), were published in ACM Transactions on Mathematical Software (TOMS) as Algorithm 680 (in Fortran, later translated into C) and Algorithm 916 by Zaghloul and Ali (in MATLAB).
Gnuplot incorporates a C implementation of the Humlicek algorithm, which is fast but relatively low precision. A free/open-source C++ implementation derived from a combination of Algorithm 680 and Algorithm 916 (using different algorithms for different z) is also available under the MIT License. This implementation is also available as a plug-in for Matlab, GNU Octave, and in Python via Scipy as scipy.special.wofz
(which was originally the TOMS 680 code, but was replaced due to copyright concerns), and it has been packaged into a C library libcerf.