Figure 1: Examples of one-loop (first-order), two-loop (second-order), and three-loop (third-order) Feynman diagrams used to calculate the fermionic self-energy $Σ(k,ω)$ (and thus the fermion lifetime) for a model of fermions interacting with a massless bosonic field. Solid lines represent fermions, dashed lines are bosons ($p$ and $l$ are fermionic and bosonic momenta, respectively). The loop expansion is expressed in powers of $1/N$, where $N$ is the number of fermionic flavors, artificially extended from $N=1$ to $N>>1$. By a naive power counting, the three-loop self-energy at zero temperature should scale as $1/N3$. In reality, some of three-loop diagrams (including the one shown) do not contain $1/N$ in the prefactor. Moreover, for this diagram $dΣ(k,ω)/dk$ diverges logarithmically when $ω$ and $k-kF$ vanish. These diagrams contain only backscattering and forward scattering and represent hidden 1D processes which, as it turned out, play a crucial role in the behavior of 2D systems. (Figure adapted from [2].)