gigMean gives the mean of the generalized inverse Gaussian
distribution, gigVar the variance, gigSkew the skewness,
gigKurt the kurtosis, and gigMode the mode. The formulae
used are as given in Jorgensen (1982),
pp. 13--17. Note that the kurtosis is the standardised fourth cumulant
or what is sometimes called the kurtosis excess. (See
Jorgensen, B. (1982).
Statistical Properties of the Generalized Inverse Gaussian
Distribution.
Lecture Notes in Statistics, Vol. 9, Springer-Verlag, New York.