Finite abelian groups are neither injective nor projective
Let
is not projective is not injective
Hints
- If you want to be sneaky, you can use this characterization of projective modules and this characterization of injective modules over a PID.
- If you would prefer a direct approach, consider the following strategy:
-
Note that a direct summand of a projective/injective module is also projective/injective
-
By the Fundamental Theorem of Finite Abelian Groups,
has a direct summand of the form for some prime and positive integer -
By considering the short exact sequence below, you can show
is neither injective nor surjective:
-