group {#sec:frg}
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The central object in the functional renormalisation group approach to quantum field theory is the scale-dependent effective action $\Gamma_k$. It generalises the effective action $\Gamma$, in the spirit of the Wilsonian RG, by introducing a cutoff scale $k$ such that $\Gamma_k$ includes only fluctuations from momentum modes with momenta larger than $k$, see [@Berges:2000ew; @Pawlowski:2005xe; @Gies:2006wv; @Schaefer:2006sr; @Braun:2011pp] for QCD-related reviews. On a technical level this is achieved by giving a momentum dependent mass to modes with momenta smaller than the scale $k$ by means of an infrared regulator function $R_k$. In this way the scale-dependent effective action $\Gamma_k$ interpolates between a microscopic action, parameterised by a finite set of parameters, at some large cutoff scale $k=\Lambda_\text{UV}$ and the full quantum effective action in the limit $k\rightarrow 0$. The evolution of $\Gamma_k$ with the RG-scale $k$ is described in terms an exact equation of one-loop structure, [@Wetterich:1992yh] $$\label{eq:floweq}
\partial_t\Gamma_k=\frac{1}{2}\text{Tr}\,
\frac{1}{\Gamma_k^{(2)}+R_k}\partial_t R_k\,.$$ Here $\Gamma_k^{(2)}$ denotes the second functional derivative with respect to the fields, $t=\log(k/\Lambda)$ with some reference scale $\Lambda$, and the trace includes a sum over all field species and internal indices as well as a momentum-space integration. Note that the flow equation is one-loop exact, higher 