Completeness of the SO(4) Extended Bethe Ansatz for the One-Dimensional   Hubbard Model
  We show how to construct a complete set of eigenstates of the hamiltonian of the one-dimensional Hubbard model on a lattice of even length $L$. This is done by using the nested Bethe Ansatz {\it and} the $SO(4)$ symmetry of the model. We discuss in detail how the counting of independent eigenstates is carried out. 
Fabian H.L. Essler, Vladimir E. Korepin, Kareljan Schoutens