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This expression is valid only for positive values of x, but it can be used in conjunction with erfc x = 2 − erfc(−x) to obtain erfc(x) for negative values. This form is advantageous in that the range of integration is fixed and finite.
The function was tabulated by Vera Faddeeva and N. N. Terentyev in 1954. [8] It appears as nameless function w(z) in Abramowitz and Stegun (1964), formula 7.1.3. The name Faddeeva function was apparently introduced by G. P. M. Poppe and C. M. J. Wijers in 1990; [9] [better source needed] previously, it was known as Kramp's function (probably after Christian Kramp).
An exGaussian random variable Z may be expressed as Z = X + Y, where X and Y are independent, X is Gaussian with mean μ and variance σ 2, and Y is exponential of rate λ. It has a characteristic positive skew from the exponential component.
Computationally, erfi(x) has the drawback that it grows roughly as exp(x^2), which quickly leads to arithmetic overflow; an alternative is to compute the Dawson function, which is essentially exp(-x^2)erfi(x) to remove this exponential factor.
The Q-function can be generalized to higher dimensions: [14] = (),where (,) follows the multivariate normal distribution with covariance and the threshold is of the form = for some positive vector > and positive constant >.
In other words, a sequence of functions is an asymptotic scale if each function in the sequence grows strictly slower (in the limit ) than the preceding function. If f {\displaystyle \ f\ } is a continuous function on the domain of the asymptotic scale, then f has an asymptotic expansion of order N {\displaystyle \ N\ } with respect to the ...
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Repeated application of the recurrence relation for the lower incomplete gamma function leads to the power series expansion: [2] (,) = = (+) (+) = = (+ +). Given the rapid growth in absolute value of Γ(z + k) when k → ∞, and the fact that the reciprocal of Γ(z) is an entire function, the coefficients in the rightmost sum are well-defined, and locally the sum converges uniformly for all ...