I would like to ask if it exists an explicit description of Aut(G), group of automorphisms of a finite group G, in particular, in case when G is abelian. In example, if G=(Z/mZ)x(Z/mZ), where m is a positive integer, how can we describe Aut(G)? Which relation we have between it and GLm(Z)? If m is prime Aut(G)=GLm(Z), but what happens for m general? In example, if m=4, I find that the cardinality of Aut(Z/4Z x Z/4Z) is 8, seeing where the generators (0 1) and (1 0) are sent. But I have the feeling that GL4(Z) should be contained in Aut(Z/4Z x Z/4Z), so I should have a problem with cardinalities.

More in general, is it sufficient to see where a minimal set of generators is sent? Maybe someone could indicate me a text where I could find a good description of such automorphism groups?