def kallen ( x , y , z ) :
    """ Kallen function, aka ``triangle'' function 
    - see https://en.wikipedia.org/wiki/K%C3%A4ll%C3%A9n_function
    """
    return x * x + y * y + z * z - 2.0 * x * y - 2.0 * y * z -  2.0 * z * x