The nested SU(N) off-shell Bethe ansatz and exact form factors
  The form factor equations are solved for an SU(N) invariant S-matrix under the assumption that the anti-particle is identified with the bound state of N-1 particles. The solution is obtained explicitly in terms of the nested off-shell Bethe ansatz where the contribution from each level is written in terms of multiple contour integrals. 
Hratchia M. Babujian, Angela Foerster and Michael Karowski