Exact result on the Mott transition in a two-dimensional model of   strongly correlated electrons
  We study the properties of a quarter-filled system of electrons on a square lattice interacting through a local repulsion $U$ and a nearest-neighbour repulsion $V$ in the limit $V=+\infty$. We identify the ground-state for $U$ large enough and show that domain walls appear below a critical value $U_c=4t/\pi$. We argue that this corresponds to a metal-insulator transition due to Mott localization. 
Frederic Mila