onal
        Optional output array for the function values

    Returns
    -------
    scalar or ndarray
        Values of the exponentially scaled Hankel function.

    Notes
    -----
    A wrapper for the AMOS [1]_ routine `zbesh`, which carries out the
    computation using the relation,

    .. math:: H^{(1)}_v(z) =
              \frac{2}{\imath\pi} \exp(-\imath \pi v/2) K_v(z \exp(-\imath\pi/2))

    where :math:`K_v` is the modified Bessel function of the second kind.
    For negative orders, the relation

    .. math:: H^{(1)}_{-v}(z) = H^{(1)}_v(z) \exp(\imath\pi v)

    is used.

    References
    ----------
    .. [1] Donald E. Amos, "AMOS, A Portable Package for Bessel Functions
           of a Complex Argument and Nonnegative Order",
           http://netlib.org/amos/
    