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Let $X/k$ be a surface (over some field), smooth except for an isolated (closed) point $x$. One may look at the punctured local ring

$X:=\mathrm{Spec}(\mathcal{O}_{X,x}) - x$.

Are there non-trivial vector bundles on $X$? If $X$ is smooth at the puncture as well, every vector bundle on $X$ must be trivial, thanks to low-dimensionality (since it must be the pullback of a vector bundle on the entire local ring).

I suspect the answer is yes, but I would be interested in explicit examples, literature on this, etc. Any advice?

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  • $\begingroup$ Denote the maximal ideal of $\mathcal{O}_{X,x}$ by $\mathfrak{m}_{X,x}$. Let $\phi:\mathcal{O}_{X,x}^{\oplus (r+1)} \to \mathfrak{m}_{X,x}$ be any surjection of $\mathcal{O}_{X,x}$-modules. The kernel of $\phi$ is locally free on $X$. It is not a free $\mathcal{O}_{X,x}$-module, since $\mathcal{O}_{X,x}/\mathfrak{m}_{X,x}$ has no finite free resolution (when $\mathcal{O}_{X,x}$ is not regular). $\endgroup$ Commented Aug 3, 2016 at 10:47

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I am just posting my comment as an answer. For a Noetherian local ring $\mathcal{O}_{X,x}$ with maximal ideal $\mathfrak{m}_{X,x}$, the $\mathcal{O}_{X,x}$-module $\mathcal{O}_{X,x}/\mathfrak{m}_{X,x}$ has a finite free resolution if and only if the local ring is regular. Thus, for every surjection of $\mathcal{O}_{X,x}$-modules, $$\phi:\mathcal{O}_{X,x}^{\oplus (r+1)} \to \mathfrak{m}_{X,x},$$ the kernel of $\phi$ is a finitely generated $\mathcal{O}_{X,x}$ that is not free, yet the restriction of the kernel of $\phi$ to $\text{Spec}(\mathcal{O}_{X,x}) \setminus \{\mathfrak{m}_{X,x}\}$ is locally free.

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  • $\begingroup$ that's a great answer, thank you! your few lines make the situation seem so dramatically clearer... it's really elegant as well. $\endgroup$
    – ol.ta
    Commented Aug 3, 2016 at 13:13

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