Exact Ground State of the 2D Hubbard Model at Half Filling for $U=0^{+}$
  We solve analytically the $N\times N$ square lattice Hubbard model for even $N$ at half filling and weak coupling by a new approach. The exact ground state wave function provides an intriguing and appealing picture of the antiferromagnetic order. Like at strong coupling, the ground state has total momentum $K_{tot}=(0,0)$ and transforms as an $s$ wave for even $N/2$ and as a $d$ wave otherwise. 
Michele Cini and Gianluca Stefanucci