$U$ independent eigenstates of Hubbard model
  Two-dimensional Hubbard model is very important in condensed matter physics. However it has not been resolved though it has been proposed for more than 50 years. We give several methods to construct eigenstates of the model that are independent of the on-site interaction strength $U$. 
Ming-Yong Ye and Xiu-Min Lin
cond-mat.str-el cond-mat.supr-con