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Let $A$ be an associative algebra, $M$ a right $A$-module. Suppose we are given an $A$-module homomorphism $M \to A$. Then we can make $M$ itself into an associative algebra via the multiplication $$ M \otimes M \to M \otimes A \to M.$$ The same construction works if $A$ and $M$ are cochain complexes, ie when $A$ is a dg algebra. In this case one can moreover prove that the construction is well defined on the level of the derived category: if $f,g \colon M \to A$ are chain maps that are equal in the derived category, then the resulting dg-algebra structures on $M$ are quasi-isomorphic.

This seems like a natural enough construction that it has been studied before. Does it have a name? Is there a reference in the literature to the claim that this is well defined in the derived category?

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  • $\begingroup$ Well, it's like thinking of an ideal as a non-unital subring, isn't it? $\endgroup$ Commented Mar 12, 2018 at 11:25
  • $\begingroup$ That's right - when $M$ is an ideal in $A$, this construction gives you back the ring structure on $M$. $\endgroup$ Commented Mar 12, 2018 at 12:43

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