N-order Darboux transformation and a spectral problem on semiaxis
  N-order Darboux transformation operator is defined on the basis of a general notion of transformation operators. Factorisation properties of this operator are studied. The Darboux transformation operator technique is applied to construct and investigate potentials with bound states at arbitrary energies for the spectral problem on semiaxis. 
Vladislav G. Bagrov, Boris F. Samsonov and L. A. Shekoyan