COMMUNICATIONS IN ALGEBRA, vol.48, no.7, pp.2894-2904, 2020 (SCI-Expanded)
A module M over a ring R is called automorphism-invariant if M is invariant under automorphisms of its injective hull E(M) and M is called co-Hopfian if each injective endomorphism of M is an automorphism. It is shown that (1) being co-Hopfian, directly-finite and having the cancelation property or the the substitution property are all equivalent conditions on automorphism-invariant modules, (2) if is an automorphism-invariant module, then M is co-Hopfian iff M-1 and M-2 are co-Hopfian, (3) if M is an automorphism-invariant module, then M is co-Hopfian if and only if E(M) is co-Hopfian. The module M is called weakly co-Hopfian if any injective endomorphism of M is essential. We also show that (4) if R is a right and left automorphism-invariant ring, then R is stably finite iff R-R or is a co-Hopfian module iff if R-n is weakly co-Hopfian as a right or left R-module for all n is an element of N.