# fields, Tools for spatial data
# Copyright 2004-2011, Institute for Mathematics Applied Geosciences
# University Corporation for Atmospheric Research
# Licensed under the GPL -- www.gpl.org/licenses/gpl.html
"Exponential" <- function(d, range = 1, alpha = 1/range, 
    phi = 1) {
    #
    # Matern covariance function transcribed from Stein's book page 31
    # nu==smoothness==.5, alpha ==  1/range
    #
    # GeoR parameters map to kappa==smoothness and phi == range
    # check for negative distances
    if (any(d < 0)) 
        stop("distance argument must be nonnegative")
    #
    return(phi * exp(-d * alpha))
}