Tridiagonal transition matrix for birth and death chain

        Returns
        -------
        P : (N,N) ndarray
            Transition matrix for birth and death chain with given
            creation and anhilation probabilities.

def transition_matrix(self):
        """Tridiagonal transition matrix for birth and death chain

        Returns
        -------
        P : (N,N) ndarray
            Transition matrix for birth and death chain with given
            creation and anhilation probabilities.

        """
        P0 = np.diag(self.r, k=0)
        P1 = np.diag(self.p[0:-1], k=1)
        P_1 = np.diag(self.q[1:], k=-1)
        return P0 + P1 + P_1