The phase diagram for the $(\lambda \phi ^{4}+\sigma \phi ^{2})_{2}$   model
  The two parameter model is reduced to a one parameter model by using simple transformations. Because the separation between different phase regions for a one parameter model is just a point, the equivalence between the two models leads to the exact equation of the line that separates the broken and un-broken phases in the $(\lambda ,\sigma)$ plane. Also, we obtain nontrivial estimates on the stability region for this model. 
E. Prodan