On geometric properties of joint invariants of Killing tensors
  We employ the language of Cartan's geometry to present a model for studying vector spaces of Killing two-tensors defined in pseudo-Riemannian spaces of constant curvature under the action of the corresponding isometry group. We also discuss geometric properties of joint invariants of Killing two-tensors defined in the Euclidean plane to formulate and prove an analogue of the Weyl theorem on joint invariants. In addition, it is shown how the joint invariants manifest themselves in the theory of superintegrable Hamiltonian systems. 
Caroline M. Adlam, Raymond G. McLenaghan, and Roman G. Smirnov