Exact Results in Quantum Theory

The most commonly used metod of derivation of quantities in quantum field theory uses Feynman diagrams and Feynman propagators. Unfortunately, when applied to gauge theories, it mostly ignored the gauge symmetry, which becomes obscured at intermediate steps, and even in the final result it's not always visible. However there exist a manifestly covariant way to make calculations within a quantum field theory, with the use of gauge-field-dependent propagators instead of the free-field Feynman propagators.I'm going to present a way to derive gauge-field dependent propagators in the form that can be used in perturbative calculations made with Feynman diagrams and show several examples of application of such propagator in Feynman diagrams of different degree of complexity.