STK_EXAMPLE_MISC01  Several correlation functions from the Matern family

 The Matern 1/2 correlation function is also known as the "exponential correla-
 tion function". This is the correlation function of an Ornstein-Ulhenbeck pro-
 cess.

 The Matern covariance function tends to the Gaussian correlation function when
 its regularity (smoothness) parameter tends to infinity.

 See also: stk_materncov_iso, stk_materncov_aniso