The Effective electromagnetic interaction in a dense fermionic medium in QED_{2+1}

The effective Lagrangian of arbitrary varying in space electromagnetic field in a dense medium is derived. It has been used for investigation of interaction between charged fermions in the medium. It is shown the possibility for the formation of metastable electron bound states in the medium when external magnetic field is applied.Comment: 6 pages, LaTeX, 1 figure (PS file), minor change