A module $M$ over a ring $R$ is an abelian group with a ring homomorphism
$$ \nu: R \to End(M) $$of $R$ into the ring of endomorphisms.
It can be thought like if you have an "action" of the ring $R$ on $M$, similar to what happen with group actions: a group homomorphism $G \to Aut(M)$.
Consequently, given an abelian group $M$, it is trivially an $End(M)$-module, and given any subring $S\subset End(M)$ we have an $S$-module structure in $M$.
Important case: simple module.
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Author of the notes: Antonio J. Pan-Collantes
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