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Let $A$ be an associative algebra over a field $k$ and let $M$ be some $A$-module. If $\mathfrak{a}$ is an ideal of $A$ which annihilates $M$, i.e. if $\mathfrak{a}M=0$, then why must every composition factor of $M$ be a composition factor of $A/\mathfrak{a}$?

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    Sounds good to me.2011-05-18

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Let E be a composition factor of M. Then E is an irreducible A-submodule of M, and since aM=0 we can consider E as an irreducible A/a-module. Since every irreducible A/a-module must occur as a composition factor in its regular representation, we are done.