docs: 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)