CLUSTER-EXPANSION APPROACH TO THE EFFECTIVE POTENTIAL IN PHI(4)(2-THEORY(1))

We apply a truncated set of dynamical equations of motion for connecte d equal-time Green functions up to the 4-point level to the investigat ion of spontaneous ground state symmetry breaking in Phi(2+1)(4) quant um field theory. Within our momentum space discretization we obtain a second order phase transition as soon as the connected 3-point functio n is included. However, an additional inclusion of the connected 4-poi nt function still shows a significant influence on the shape of the ef fective potential and the critical coupling.