docs: def find_omega_min(omega_levelu, Neu, Nl, xiu):
      r"""Find the smallest transition frequency for each field.
  
      >>> Ne = 6
      >>> Nl = 2
      >>> omega_level = [0.0, 100.0, 100.0, 200.0, 200.0, 300.0]
      >>> xi = np.zeros((Nl, Ne, Ne))
      >>> coup = [[(1, 0), (2, 0)], [(3, 0), (4, 0), (5, 0)]]
      >>> for l in range(Nl):
      ...     for pair in coup[l]:
      ...         xi[l, pair[0], pair[1]] = 1.0
      ...         xi[l, pair[1], pair[0]] = 1.0
  
      >>> aux = define_simplification(omega_level, xi, Nl)
      >>> u, invu, omega_levelu, Neu, xiu = aux
      >>> find_omega_min(omega_levelu, Neu, Nl, xiu)
      ([100.0, 200.0], [1, 2], [0, 0])
  
      """
      omega_min = []; iu0 = []; ju0 = []
      for l in range(Nl):
          omegasl = []
          for iu in range(Neu):
              for ju in range(iu):
                  if xiu[l, iu, ju] == 1:
                      omegasl += [(omega_levelu[iu]-omega_levelu[ju], iu, ju)]
          omegasl = list(sorted(omegasl))
          omega_min += [omegasl[0][0]]
          iu0 += [omegasl[0][1]]
          ju0 += [omegasl[0][2]]
  
      return omega_min, iu0, ju0