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Modified Bessel function of noninteger order.


double v, x, y, iv();
y = iv(v, x);


Returns the modified Bessel function of order v of the argument. If x is negative, v must be integer-valued.
The function is defined as Iv(x) = Jv(ix).  Here, it is computed in terms of the confluent hypergeometric function, according to the formula:
               v  -x
  Iv(x) = (x/2)  e   hyperg(v+0.5, 2v+1, 2x) / gamma(v+1)

  If v is a negative integer, then v is replaced by -v.


Tested at random points (v, x), with v between 0 and  30, x between 0 and 28.

                       Relative error:
  arithmetic   domain     # trials      peak         rms
     DEC       0,30          2000      3.1e-15     5.4e-16
     IEEE      0,30         10000      1.7e-14     2.7e-15

Accuracy is diminished if v is near a negative integer.