The Smorodinsky-Winternitz potential revisited
  We employ joint invariants of Killing two-tensors defined in the Euclidean plane to characterize the Smorodinsky-Winternitz potential and explain the geometric meaning of its arbitrary parameters. In addition, we verify for which values of the arbitrary parameter $k$ the Tremblay-Turbiner-Winternitz potential is multi-separable. 
Roman G. Smirnov, Amelia L. Yzaguirre
math-ph math.MP