Twisting singular solutions of Bethe's equations
  The Bethe equations for the periodic XXX and XXZ spin chains admit singular solutions, for which the corresponding eigenvalues and eigenvectors are ill-defined. We use a twist regularization to derive conditions for such singular solutions to be physical, in which case they correspond to genuine eigenvalues and eigenvectors of the Hamiltonian. 
Rafael I. Nepomechie and Chunguang Wang
math-ph hep-th math.MP