omega2 < omega < omega3

def _n_2(self):
        """omega2 < omega < omega3"""
        return (self._f(3, 1) * self._f(2, 1) +
                self._f(3, 0) * self._f(1, 3) * self._f(2, 1) +
                self._f(3, 0) * self._f(2, 0) * self._f(1, 2))