The Nested Off-shell Bethe ansatz and O(N) Matrix Difference Equations
  A system of O(N)-matrix difference equations is solved by means of the off-shell version of the nested algebraic Bethe ansatz. In the nesting process a new object, the $\Pi$-matrix, is introduced to overcome the complexities of the O(N) group structure. The proof of the main theorem is presented in detail. In particular, the cancellation of all unwanted terms is shown explicitly. The highest weight property of the solutions is proved. 
H. Babujian, A. Foerster and M. Karowski
math-ph hep-th math.MP