Singularities in the Vertex Function in Two-Dimensional Tight-Binding Fermi Systems

The singularities in the vertex function are studied within the ladder approximation for a non-half-filled two-dimensional tight- binding Fermi system. The location of all the possible poles of the vertex function has been clarified for both repulsive and attractive interacting cases.. The scattering phase shift is examined and found not always zero at Fermi energy. The Kohn-lut tinger-like singularity is also briefly discussed.