knModified Bessel function, third kind, integer order.
double x, y, kn();
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.
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.