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Modified Bessel function, third kind, integer order.


double x, y, kn();
int n;
y = kn(n, x);


Returns the modified Bessel function of the third kind, of order n of the argument.

The range is partitioned into the two intervals [0,9.55] and (9.55, infinity). An ascending power series is used in the low range, and an asymptotic expansion in the high range.

                      Relative error:
arithmetic   domain     # trials      peak         rms
    DEC       0,30         3000       1.3e-9      5.8e-11
    IEEE      0,30        90000       1.8e-8      3.0e-10

Error is high only near the crossover point x = 9.55 between the two expansions used.