Comparing Different Improvement Programs for the N-Vector Model
  We discuss the connection between various types of improved actions in the context of the two-dimensional sigma-model. We also discuss spectrum-improved actions showing that these actions do not have any improved behaviour. An O(a^2) on-shell improved action with all couplings defined on a plaquette and satisfying reflection positivity is also explicitly constructed. 
Sergio Caracciolo and Andrea Pelissetto