The invariance for the equation of fast diffusion in the 2D coordinate space
has been proved, and its reduction to the 1D (with respect to the spatial
variable) analog is demonstrated. On the basis of these results, new exact
multi-dimensional solutions, which are dependent on arbitrary harmonic
functions, are constructed. As a result, new exact solutions of the well-known
Liouville equation - the steady-state analog for the fast diffusion equation
with the linear source - have been obtained. Some generalizations for the
systems of quasilinear parabolic equations, as well as systems of elliptic
equations with Poisson interaction, which are applied in the theory of
semiconductors, are considered.