%N bessi1 %F $MIR/src/subs/bessel.for
%D Compute modified Bessel function I1
%P nebk
%: utilities, numbers
%B
      double precision function bessi1 (x)

      implicit none
      double precision x

     Compute modified Bessel function I1(x) with polynomial
     approximation.   Valid for all arguments.
     Error is < 8e-9 for |x| < 3.75, else < 2.2e-7

     Algorithm from Handbook of Mathematical Functions, P 378

  Input
    x       Argument
  Output
    bessi1  Result
