## 37.42 Zariski's Main Theorem

In this section we prove Zariski's main theorem as reformulated by Grothendieck. Often when we say “Zariski's main theorem” in this content we mean either of Lemma 37.42.1, Lemma 37.42.2, or Lemma 37.42.3. In most texts people refer to the last of these as Zariski's main theorem.

We have already proved the algebraic version in Algebra, Theorem 10.123.12 and we have already restated this algebraic version in the language of schemes, see Morphisms, Theorem 29.55.1. The version in this section is more subtle; to get the full result we use the étale localization techniques of Section 37.40 to reduce to the algebraic case.

Lemma 37.42.1. Let $f : X \to S$ be a morphism of schemes. Assume $f$ is of finite type and separated. Let $S'$ be the normalization of $S$ in $X$, see Morphisms, Definition 29.53.3. Picture:

\[ \xymatrix{ X \ar[rd]_ f \ar[rr]_{f'} & & S' \ar[ld]^\nu \\ & S & } \]

Then there exists an open subscheme $U' \subset S'$ such that

$(f')^{-1}(U') \to U'$ is an isomorphism, and

$(f')^{-1}(U') \subset X$ is the set of points at which $f$ is quasi-finite.

**Proof.**
By Morphisms, Lemma 29.55.2 the subset $U \subset X$ of points where $f$ is quasi-finite is open. The lemma is equivalent to

$U' = f'(U) \subset S'$ is open,

$U = f^{-1}(U')$, and

$U \to U'$ is an isomorphism.

Let $x \in U$ be arbitrary. We claim there exists an open neighbourhood $f'(x) \in V \subset S'$ such that $(f')^{-1}V \to V$ is an isomorphism. We first prove the claim implies the lemma. Namely, then $(f')^{-1}V \cong V$ is both locally of finite type over $S$ (as an open subscheme of $X$) and for $v \in V$ the residue field extension $\kappa (v)/\kappa (\nu (v))$ is algebraic (as $V \subset S'$ and $S'$ is integral over $S$). Hence the fibres of $V \to S$ are discrete (Morphisms, Lemma 29.20.2) and $(f')^{-1}V \to S$ is locally quasi-finite (Morphisms, Lemma 29.20.8). This implies $(f')^{-1}V \subset U$ and $V \subset U'$. Since $x$ was arbitrary we see that (a), (b), and (c) are true.

Let $s = f(x)$. Let $(T, t) \to (S, s)$ be an elementary étale neighbourhood. Denote by a subscript ${}_ T$ the base change to $T$. Let $y = (x, t) \in X_ T$ be the unique point in the fibre $X_ t$ lying over $x$. Note that $U_ T \subset X_ T$ is the set of points where $f_ T$ is quasi-finite, see Morphisms, Lemma 29.20.13. Note that

\[ X_ T \xrightarrow {f'_ T} S'_ T \xrightarrow {\nu _ T} T \]

is the normalization of $T$ in $X_ T$, see Lemma 37.18.2. Suppose that the claim holds for $y \in U_ T \subset X_ T \to S'_ T \to T$, i.e., suppose that we can find an open neighbourhood $f'_ T(y) \in V' \subset S'_ T$ such that $(f'_ T)^{-1}V' \to V'$ is an isomorphism. The morphism $S'_ T \to S'$ is étale hence the image $V \subset S'$ of $V'$ is open. Observe that $f'(x) \in V$ as $f'_ T(y) \in V'$. Observe that

\[ \xymatrix{ (f'_ T)^{-1}V' \ar[r] \ar[d] & (f')^{-1}(V) \ar[d] \\ V' \ar[r] & V } \]

is a fibre square (as $S'_ T \times _{S'} X = X_ T$). Since the left vertical arrow is an isomorphism and $\{ V' \to V\} $ is a étale covering, we conclude that the right vertical arrow is an isomorphism by Descent, Lemma 35.22.17. In other words, the claim holds for $x \in U \subset X \to S' \to S$.

By the result of the previous paragraph we may replace $S$ by an elementary étale neighbourhood of $s = f(x)$ in order to prove the claim. Thus we may assume there is a decomposition

\[ X = V \amalg W \]

into open and closed subschemes where $V \to S$ is finite and $x \in V$, see Lemma 37.40.4. Since $X$ is a disjoint union of $V$ and $W$ over $S$ and since $V \to S$ is finite we see that the normalization of $S$ in $X$ is the morphism

\[ X = V \amalg W \longrightarrow V \amalg W' \longrightarrow S \]

where $W'$ is the normalization of $S$ in $W$, see Morphisms, Lemmas 29.53.10, 29.44.4, and 29.53.12. The claim follows and we win.
$\square$

slogan
Lemma 37.42.2. Let $f : X \to S$ be a morphism of schemes. Assume $f$ is quasi-finite and separated. Let $S'$ be the normalization of $S$ in $X$, see Morphisms, Definition 29.53.3. Picture:

\[ \xymatrix{ X \ar[rd]_ f \ar[rr]_{f'} & & S' \ar[ld]^\nu \\ & S & } \]

Then $f'$ is a quasi-compact open immersion and $\nu $ is integral. In particular $f$ is quasi-affine.

**Proof.**
This follows from Lemma 37.42.1. Namely, by that lemma there exists an open subscheme $U' \subset S'$ such that $(f')^{-1}(U') = X$ and $X \to U'$ is an isomorphism. In other words, $f'$ is an open immersion. Note that $f'$ is quasi-compact as $f$ is quasi-compact and $\nu : S' \to S$ is separated (Schemes, Lemma 26.21.14). It follows that $f$ is quasi-affine by Morphisms, Lemma 29.13.3.
$\square$

reference
Lemma 37.42.3 (Zariski's Main Theorem). Let $f : X \to S$ be a morphism of schemes. Assume $f$ is quasi-finite and separated and assume that $S$ is quasi-compact and quasi-separated. Then there exists a factorization

\[ \xymatrix{ X \ar[rd]_ f \ar[rr]_ j & & T \ar[ld]^\pi \\ & S & } \]

where $j$ is a quasi-compact open immersion and $\pi $ is finite.

**Proof.**
Let $X \to S' \to S$ be as in the conclusion of Lemma 37.42.2. By Properties, Lemma 28.22.13 we can write $\nu _*\mathcal{O}_{S'} = \mathop{\mathrm{colim}}\nolimits _{i \in I} \mathcal{A}_ i$ as a directed colimit of finite quasi-coherent $\mathcal{O}_ X$-algebras $\mathcal{A}_ i \subset \nu _*\mathcal{O}_{S'}$. Then $\pi _ i : T_ i = \underline{\mathop{\mathrm{Spec}}}_ S(\mathcal{A}_ i) \to S$ is a finite morphism for each $i$. Note that the transition morphisms $T_{i'} \to T_ i$ are affine and that $S' = \mathop{\mathrm{lim}}\nolimits T_ i$.

By Limits, Lemma 32.4.11 there exists an $i$ and a quasi-compact open $U_ i \subset T_ i$ whose inverse image in $S'$ equals $f'(X)$. For $i' \geq i$ let $U_{i'}$ be the inverse image of $U_ i$ in $T_{i'}$. Then $X \cong f'(X) = \mathop{\mathrm{lim}}\nolimits _{i' \geq i} U_{i'}$, see Limits, Lemma 32.2.2. By Limits, Lemma 32.4.16 we see that $X \to U_{i'}$ is a closed immersion for some $i' \geq i$. (In fact $X \cong U_{i'}$ for sufficiently large $i'$ but we don't need this.) Hence $X \to T_{i'}$ is an immersion. By Morphisms, Lemma 29.3.2 we can factor this as $X \to T \to T_{i'}$ where the first arrow is an open immersion and the second a closed immersion. Thus we win.
$\square$

Lemma 37.42.4. With notation and hypotheses as in Lemma 37.42.3. Assume moreover that $f$ is locally of finite presentation. Then we can choose the factorization such that $T$ is finite and of finite presentation over $S$.

**Proof.**
By Limits, Lemma 32.9.8 we can write $T = \mathop{\mathrm{lim}}\nolimits T_ i$ where all $T_ i$ are finite and of finite presentation over $Y$ and the transition morphisms $T_{i'} \to T_ i$ are closed immersions. By Limits, Lemma 32.4.11 there exists an $i$ and an open subscheme $U_ i \subset T_ i$ whose inverse image in $T$ is $X$. By Limits, Lemma 32.4.16 we see that $X \cong U_ i$ for large enough $i$. Replacing $T$ by $T_ i$ finishes the proof.
$\square$

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