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Modified Bessel function of order one.


double x, y, i1();
y = i1(x);


Returns the modified Bessel function of order one of the argument.
The function is defined as i1(x) = -i j1(ix).
The range is partitioned into the two intervals [0,8] and (8, infinity).  Chebyshev polynomial expansions are employed in each interval.

                      Relative error:
arithmetic   domain     # trials      peak         rms
    DEC       0, 30        3400       1.2e-16     2.3e-17
    IEEE      0, 30       30000       1.9e-15     2.1e-16