Given a policy `sigma`, return the T_sigma operator.

        Parameters
        ----------
        sigma : array_like(int, ndim=1)
            Policy vector, of length n.

        Returns
        -------
        callable
            The T_sigma operator.

def T_sigma(self, sigma):
        """
        Given a policy `sigma`, return the T_sigma operator.

        Parameters
        ----------
        sigma : array_like(int, ndim=1)
            Policy vector, of length n.

        Returns
        -------
        callable
            The T_sigma operator.

        """
        R_sigma, Q_sigma = self.RQ_sigma(sigma)
        return lambda v: R_sigma + self.beta * Q_sigma.dot(v)