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I have a regular map $f : X \to Y$ and a subvariety $V \subset X$ (assume everything is smooth). I'd like to study this map in an infinitesimal neighborhood of $V$, but I'm not sure of the right notion of "local coordinates" in this setting.

I suppose I want to look at the completion of the local ring $\widehat{\mathcal O}_{X,V}$ -- is this isomorphic to $K(V)[[x_1,\ldots,x_k]]$, where $K(V)$ is the function field of $V$ and $k$ is the codimension? My impression is that Cohen structure theorem says something like this, but I haven't found this spelled out explicitly. I'd also expect the normal bundle of $V$ to make an appearance here. Question: Does there exist an isomorphism of the form I want? And how is the possible non-triviality of the normal bundle of $V$ reflected there?

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  • $\begingroup$ Take a look at EGA 4 $\S 16.1$. $\endgroup$ – Reimundo Heluani Nov 4 '14 at 22:08
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I presume that by $\widehat{\mathcal{O}}_{X, V}$ you mean the ring obtained by completing the local ring at the generic point of $V$ at its maximal ideal.

You're right that your description ($\widehat{\mathcal{O}}_{X, V}=K(V)[[x_1, ..., x_k]]$) follows immediately from the Cohen structure theorem (see e.g. 10.149.8 here); in fact it is not necessary that everything be smooth, just that the local ring at the generic point of $V$ is regular. Indeed, any complete regular local ring containing a field is a power series ring over a field. In this case, your ring contains $K(V)$, and $K(V)$ is the maximal field it contains (because it is isomorphic to its residue field, for example). This is prove on page 11 here, for example. The fact that $k$ equals the codimension follows by considering the Krull dimension of the local ring at the generic point of $V$.

But I suspect that this is not what you mean to look for; this is something like "an infinitesimal neighborhood of the generic point of $V$". I suspect you really mean to look at the formal scheme obtained by completing $X$ at $V$. This is the topologically locally ringed space whose underlying topological space is the same as that of $V$, but whose sheaf of rings is given by taking the ring of functions on $X$ completed at the ideal defining $V$. This is a much better notion of an "infinitesimal neighborhood of $V$"; I usually tell people it is something like an "infinitesimal tubular neighborhood." Formal schemes are described briefly in Hartshorne, but as in Reimundo Heluani's comment, EGA is a much better reference (I think this is EGA $\text{III}_1$, which is a comparatively gentle part of EGA).

In particular, the formal scheme obtained this way, which I'll call $\hat V$, remembers the normal bundle of $V$ in $X$; it is the same as the normal bundle of $V$ in $\hat V$, appropriately defined! In certain very nice cases, it is isomorphic to the formal scheme obtained by completing the total space of the normal bundle of $V$ in $X$ at its zero section, but that is not always the case. See e.g. the comments to this Mathoverflow answer of Jason Starr's.

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