def drude(x, x0=4.59, gamma=0.90, **extras):
    """Drude profile for the 2175AA bump.

    :param x:
        Inverse wavelength (inverse microns) at which values for the drude
        profile are requested.

    :param gamma:
        Width of the Drude profile (inverse microns).

    :param x0:
       Center of the Drude profile (inverse microns).

    :returns k_lambda:
       The value of the Drude profile at x, normalized such that the peak is 1.
     """
    #return (w * gamma)**2 / ((w**2 - w0**2)**2 + (w * gamma)**2)
    return (x*gamma)**2 / ((x**2 - x0**2)**2 + (x * gamma)**2)