BESSELKTILDE is the jOceans module of jLab.

 BESSELKTILDE  K-type Bessel function after factoring off exponential decay.
 
    KTILDE=BESSELKTILDE(NU,Z) returns the modified Bessel function of the
    second kind of order NU at argument Z, after factoring off the 
    asymptotic behavior of EXP(-Z).
 
    BESSELKTILDE is useful for products of modified Bessel functions in
    which the exponential behaviors cancel, but that cannot be evaluated
    directly because of numerical overflow.
 
    BESSELITILDE(NU,Z,N) use a summation truncated at N terms. The default
    behavior uses N=30 and is highly accurate. 
 
    See 10.40.2 of https://dlmf.nist.gov/10.40.
 
    This is low-level code used by WINDTRANS using an algorithm described 
    in Lilly and Elipot (2021).
 
    See also BESSELITILDE.
 
    'besselktilde --t' runs a test.
 
    Usage: K=besselktilde(nu,z);
           K=besselktilde(nu,z,nterms);
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    This is part of JLAB --- type 'help jlab' for more information
    (C) 2019--2021 J.M. Lilly --- type 'help jlab_license' for details