@classmethod
def _lovelace_function(cls, r):
    """The L(r) function as defined by (Lovelace et al 1999) & (Li et al 2000).

        Rossby Wave Instability (RWI) is characterized by the presence
        of a local extremum in this function.
        """
    return cls._entropy(r) ** (2 / cls.gamma) / (2 * cls._vortensity(r))
