MATERNFUN is the jMatern module of jLab.

 MATERNFUN  Returns the Matern function.  
 
    MATERNFUN is a low-level function called by MATERNCOV.
 
    M=MATERNFUN(ALPHA,X) returns the value of the Matern function of order
    ALPHA at location X, with ALPHA>1/2.  The Matern function is defined as
 
       M(X)=C*|X|^(ALPHA-1/2) K_(ALPHA-1/2)(|X|)
    
    where K_ALPHA is the modified Bessel function of the second kind of 
    order ALPHA, and C=2/GAMMA(ALPHA-1/2)./2^(ALPHA-1/2) is a normalizing 
    constant. This definition implies M(0)=1.
 
    For the special case of ALPHA=1/2, the GAMMA function in C is omitted
    in order to avoid developing a singularity. This change is acceptable 
    because this function is used only in ratios for ALPHA<=1/2.  
 
    'maternfun --f' generates a sample figure.
 
    Usage:  M=maternfun(alpha,x);
    __________________________________________________________________
    This is part of JLAB --- type 'help jlab' for more information
    (C) 2013--2017 J.M. Lilly --- type 'help jlab_license' for details