cubic.cov.R
# 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
cubic.cov <- function(x1, x2, theta = 1, C = NA, marginal = FALSE) {
# comments in Exp.simple.cov for more details about the
# required parts of this covariance
if (is.matrix( x1)){
if(ncol(x1) != 1){
stop(" x is a matrix this is a 1-d covariance")}}
if (missing(x2)) {
x2 <- x1
}
# local function
fun.temp <- function(u, v) {
1+ ifelse(u < v, v * (u^2)/2 - (u^3)/6, u * (v^2)/2 - (v^3)/6)
}
if (is.na(C[1]) & !marginal) {
# cross covariance matrix
return(outer(c(x1), c(x2), FUN = fun.temp))
}
if (!is.na(C[1])) {
# product of cross covariance with a vector
return(outer(c(x1), c(x2), FUN = fun.temp) %*% C)
}
if (marginal) {
# marginal variance
return((x1^3)/3)
}
}