Tridiagonal transition matrix for birth and death chain

        Returns
        -------
        P : (N,N) scipy.sparse matrix
            Transition matrix for birth and death chain with given
            birth and death probabilities.

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

        Returns
        -------
        P : (N,N) scipy.sparse matrix
            Transition matrix for birth and death chain with given
            birth and death probabilities.

        """
        P = diags([self.q[1:], self.r, self.p[0:-1]], [-1, 0, 1])
        return P