General exponential correlation function.
If n > 1 and length(theta) = 1, then the model is isotropic:
theta_j = theta[1], j=1,...,n; theta_[n+1] = theta[2].
correxpg(theta, d, ret = "all")parameters in the correlation function
m*n matrix with differences between given data points
A string. If set to "all" or "dr", the derivative of r (dr) will be returned, else dr is NA.
returns a list with two elements:
rcorrelation
drm*n matrix with the Jacobian of r at x. It is
assumed that x is given implicitly by d[i,] = x - S[i,],
where S[i,] is the i'th design site.