def dm_smear(dm, BW, center_freq):
    """
    dm_smear(dm, BW, center_freq):
        Return the smearing in sec caused by a 'dm' over a bandwidth
        of 'BW' MHz centered at 'center_freq' MHz.
    """
    return dm * BW / (0.0001205 * center_freq * center_freq * center_freq)