Quasilinear equations with exponential growth for H\"ormander $q$-sub-Laplacians on stratified Lie groups

We prove the existence of positive weak solutions for a quasilinear equation with exponential nonlinearity on arbitrary stratified Lie groups for \emph{horizontal $q$-Laplacians,} also called $q$-sub-Laplacians on these groups.

The nonlinearity combines a concave term with an exponential growth that can be subcritical, critical, or supercritical with respect to the Trudinger--Moser inequality for Lorentz spaces on stratified Lie groups.

We prove a version of this inequality in this paper.

Since the problem is not variational in the natural energy space, classical minimisation or critical-point techniques do not apply directly.

Positive solutions to the resulting semilinear equation are then obtained via a carefully designed Galerkin method applied to this setting.

Our main theorem recovers previous known results on $\mathbb{R}^n$ in the complete range $q\in (1,\infty),$ and the previous known result on the Heisenberg group $\mathbb{H}^n$ for $q=2n+2,$ and is extended here in the full range $q\in (1,\infty)$.

Other particular and fundamental cases, including the Engel group, are discussed.