def RQ_sigma(self, sigma):
        """
        Given a policy `sigma`, return the reward vector `R_sigma` and
        the transition probability matrix `Q_sigma`.

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

        Returns
        -------
        R_sigma : ndarray(float, ndim=1)
            Reward vector for `sigma`, of length n.

        Q_sigma : ndarray(float, ndim=2)
            Transition probability matrix for `sigma`, of shape (n, n).

        """
        if self._sa_pair:
            sigma = np.asarray(sigma)
            sigma_indices = np.empty(self.num_states, dtype=int)
            _find_indices(self.a_indices, self.a_indptr, sigma,
                          out=sigma_indices)
            R_sigma, Q_sigma = self.R[sigma_indices], self.Q[sigma_indices]
        else:
            R_sigma = self.R[np.arange(self.num_states), sigma]
            Q_sigma = self.Q[np.arange(self.num_states), sigma]

        return R_sigma, Q_sigma