docs: 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