periodic.plane.3d.r
"periodic.plane.3d" <-
function (x1, x2, a = 0, b = 365, theta = 1)
{
cov.per1d <- periodic.cov.1d(x1[, 1], x2[, 1], a, b)
cov.vert <- exp.cov(x1[, c(2, 3)], x2[, c(2, 3)], theta = theta)
cov <- cov.per1d * cov.vert
return(cov)
}