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Let $R = \mathbb{C}[x,y]$ and $V = \mathbb{C}x\oplus\mathbb{C}y$. Then, the Koszul resolution of $R$ (as an $R$-bimodule) is given by \begin{align*} 0\to R\otimes_{\mathbb{C}}\wedge^2V\otimes_{\mathbb{C}} R& \xrightarrow{\partial_2} R\otimes_{\mathbb{C}}V\otimes_{\mathbb{C}} R \xrightarrow{\partial_1} R\otimes_{\mathbb{C}} R \xrightarrow{\mathrm{mult}} R \to 0\,,\\ \partial_1(1\otimes v\otimes 1) &= 1\otimes v - v\otimes 1\,,\\ \partial_2(1\otimes x\wedge y\otimes 1) &= 1\otimes x\otimes y + x\otimes y\otimes1 - 1\otimes y\otimes x - y\otimes x\otimes 1\,. \end{align*}

I want to know whether this resolution is minimal (i.e. each step is a projective cover). How can I (dis)prove this?

Since $R$ is not an Artinian ring, I'm not sure even a minimal resolution exists. Nonetheless, it seems somewhat plausible. Any comments or references on other general results are appreciated.

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    $\begingroup$ Unless I'm missing something very subtle in your question, graded Nakayama lemma says that epimorphism between finitely generated modules over a connected graded algebra with 0th degree = base field is superfluous iff it is iso modulo maximal ideal. Hence projective covers exist. $\endgroup$
    – Denis T
    Commented Apr 20 at 20:11
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    $\begingroup$ Sorry, but it's not clear to me. If $f:M\to N$ is an epimorphism of graded f.g. modules and an iso modulo the maximal ideal, $\mathrm{Ker}f + H = M$ implies $H=M$ for a graded submodule $H$ by graded Nakayama's Lemma, but I'm not sure for a general submodule $H$. Am I missing something? $\endgroup$
    – Qwert Otto
    Commented Apr 21 at 5:58

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In the category of ungraded bimodules, the multiplication map $R\otimes_\mathbb{C}R\to R$ is not a projective cover. For example, the proper sub-bimodule of $R\otimes_\mathbb{C}R$ generated by $1\otimes1+x\otimes1-1\otimes x$ surjects onto $R$.

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