in the case of a negative KPZ non-linearity, [*i.e.*]{} in the MN1 class, where a standard mean-field approximation yields qualitatively correct results (see [@Review; @Lisboa] and references therein).

Field theoretical considerations
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The Langevin equation for MN1 is known to be super-renormalizable, [*i.e.*]{} Feynman diagrams can be computed to all orders and resummated. This does not imply that critical exponents can be computed in all the cases, as the renormalization group flow-equation has runaway trajectories supposed to converge to a [*strong coupling fixed point*]{}. But at least, the correct phase diagram, including strong and weak coupling fixed points can be obtained.

For the MN2 the situation is far more complicated, as can be [ *a priori*]{} anticipated given the failure of mean-field approaches. The extra term, $ (\nabla n)^2 /n $, being singular in $n$ precludes the use of perturbative expansions around $n=0$. Given the lack of a non-perturbative approach to the KPZ and MN strong coupling fixed points, there is not much we can add to this section, except that there is a promising attempt to tackle this and related KPZ-like problems. There is a formalism, developed by Fogedby, aimed at developing a strong coupling theory for KPZ based 