def shorten_authors(auth_string, display_auth = 5):
    """
    Shortens article author list. By convention first display_auth-1 authors and 
    the last author are shown. The rest is replaced by '...'.

    Parameters
    ----------
    auth_string : str
        Article authors separated by comma.
    display_auth : int, optional
        Maximum number of autors to display. The default is 5.

    Returns
    -------
    str
        Authors with middle ones substituted by '...'.

    """
    s = auth_string.split(',')
    s = [x.strip() for x in s]
    if len(s) >= (display_auth+1):
        s = s[0:(display_auth-1)] + ["..."] + [s[-1]]
    return ", ".join(s)