  Using the basic Lie symmetry method, we find the most general Lie point
symmetries group of the $\nabla u=f(u)$ Poisson's equation, which has a
subalgebra isomorphic to the $3-$dimensional special Euclidean group ${\rm
SE}(3)$ or group of rigid motions of ${\Bbb R}^3$. Looking the adjoint
representation of ${\rm SE}(3)$ on its Lie algebra $\goth{se}(3)$, we will find
the complete optimal system of its subalgebras. This latter provides some
properties of solutions for the Poisson's equation.
