The existence of a quantum phase transition in a Hubbard model on a   square lattice
  A novel canonical transformation is offered as the mean for studying properties of a system of strongly correlated electrons. As an example of the utility of the transformation, it is used to demonstrate the existence of a quantum phase transition in a Hubbard model on a square lattice. An Appendix presents two cases with a negative result. 
Valentin Voroshilov
cond-mat.str-el