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Let $A$ be a nuclear Frechet algebra with unit. Let $M$ be a right Frechet $A$-module and $N$ be a left Frechet $A$-module. Both $M$ and $N$ are assumed to be non-degenerate. We can define the projective tensor product $M\hat{\otimes}_A N$ as in The Homology of Banach and Topological Algebras Chapter II, $4.1. Actually we have another, inductive tensor product, but these two construction coincide for Frechet modules.

On the other hand, we can forget the topologies and consider the algebraic tensor product $M\otimes_A N$. In general $M\hat{\otimes}_A N\not\cong M\otimes_A N$. Nevertheless, if both $M$ and $N$ are finitely generated projective $A$-modules, which means they are direct summands of finite direct sums of $A$, then it is clear that $M\hat{\otimes}_A N\cong M\otimes_A N$.

My question is: if we only assume $M$ is finitely generated and projective, do we still have $M\hat{\otimes}_A N\cong M\otimes_A N$? If not, do we have a counter-example?

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    $\begingroup$ Think about the basic case: do we have $A\hat\otimes _AN\simeq A\otimes_AN$? $\endgroup$
    – WhatsUp
    Commented Dec 19, 2019 at 5:52
  • $\begingroup$ @WhatsUp Yes. the non-degeneracy condition means that $1\cdot n=n$. Then it is clear that $A\otimes_A N\cong N$ and by "The Homology of Banach and Topological Algebras" Page 122 Equation 1, $A\hat{\otimes}_A N\cong N$ too. The same result works for any finite direct sums of $A$ as well. $\endgroup$ Commented Dec 19, 2019 at 16:59

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