  We provide an easily checkable algebraic characterization of positive
expansivity for Additive Cellular Automata over a finite abelian group. First
of all, an easily checkable characterization of positive expansivity is
provided for the non trivial subclass of Linear Cellular Automata over the
alphabet $(\Z/m\Z)^n$. Then, we show how it can be exploited to decide positive
expansivity for the whole class of Additive Cellular Automata over a finite
abelian group.
