Gauge Symmetry and its Implications for the Schwinger-Dyson Equations
  Gauge theories have been a cornerstone of the description of the world at the level of the fundamental particles. The Lagrangian or the action describing the corresponding interactions is invariant under certain gauge transformations. This symmetry is reflected in terms of the Ward-Green-Takahashi (or the Slavnov-Taylor) identities which relate various Green functions among each other, and the Landau-Khalatnikov-Fradkin transformations which relate a Green function in a particular gauge to it in an arbitrary covariant one. As an outcome, all physical observables should be independent of the choice of the covariant gauge parameter. The most systematic scheme to solve quantum field theories (QFT) is perturbation theory where the above-mentioned identities are satisfied at every order of approximation. This feature is exploited highly usefully as a verification of the results obtained after time- and effort-consuming exercises. As it stands, not all natural phenomena are realized in the perturbative regime of QFTs. Therefore, one is inevitably led to make efforts to solve these theories in a non perturbative fashion. A natural starting point for such studies in the continuum are the Schwinger-Dyson Equations (SDE). One of the most undesirable features associated with their non perturbative truncation is the loss of highly and rightly esteemed gauge invariance rendering the predictions less reliable. Arguably, the only hope we have to recuperate the credibility of the results is to employ only such truncation schemes which yield predictions that match onto the blindly reliable perturbative ones at every order of approximation. It is a highly difficult task but not impossible. 
A. Bashir and A. Raya