Computing the Powered Exponential covariance function for a scalar distance.
single_pexp(d, sigsq, phi, nu)a scalar representing the (exponential) covariance between two
observations d apart of each other.
a scalar representing the distance on which it is desired to evaluate the covariance function.
the \(\sigma^2\) parameter from the Exponential covariance function.
the \(\phi\) parameter from the Exponential covariance function, controls the range of the spatial dependence.
the \(\nu \in (0, 2]\) parameter representing the "power"
single_exp, single_matern,
single_matern3, single_matern5,
mat_cov