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Modified Bessel function, third kind, order zero.


double x, y, k0();
y = k0(x);


Returns the modified Bessel function of the third kind of order zero of the argument.
The range is partitioned into the two intervals [0,8] and (8, infinity).  Chebyshev polynomial expansions are employed in each interval.


Tested at 2000 random points between 0 and 8.  Peak absolute error (relative when K0 > 1) was 1.46e-14; rms, 4.26e-15.

                       Relative error:
  arithmetic   domain     # trials      peak         rms
     DEC       0, 30        3100       1.3e-16     2.1e-17
     IEEE      0, 30       30000       1.2e-15     1.6e-16


   message       condition      value returned
   domain          x <= 0          MAXNUM