The mass gap and vacuum energy of the Gross-Neveu model via the 2PPI   expansion
  We introduce the 2PPI (2-point-particle-irreducible) expansion, which sums bubble graphs to all orders. We prove the renormalizibility of this summation. We use it on the Gross-Neveu model to calculate the mass gap and vacuum energy. After an optimization of the expansion, the final results are qualitatively good. 
David Dudal and Henri Verschelde