37.55 Stratifying a morphism
Let $f : X \to S$ be a finitely presented morphism of quasi-compact and quasi-separated schemes. In Section 37.54 we have seen that we can stratify $S$ such that $X$ is flat over the strata. In this section look for stratifications of both $S$ and $X$ such that we obtain smooth strata; this won't quite work and we'll need a base change by finite locally free morphisms as well.
Lemma 37.55.1. Let $f : X \to S$ be a morphism of schemes of finite presentation. Let $\eta \in S$ be a generic point of an irreducible component of $S$. Assume $S$ is reduced. Then there exist
an open subscheme $U \subset S$ containing $\eta $,
a surjective, universally injective, finite locally free morphism $V \to U$,
a $t \geq 0$ and closed subschemes
\[ X \times _ S V \supset Z_0 \supset Z_1 \supset \ldots \supset Z_ t = \emptyset \]
such that $Z_ i \to X \times _ S V$ is defined by a finite type ideal sheaf, $Z_0 \subset X \times _ S V$ is a thickening, and such that the morphism $Z_ i \setminus Z_{i + 1} \to V$ is smooth.
Proof.
It is clear that we may replace $S$ by an open neighbourhood of $\eta $ and $X$ by the restriction to this open. Thus we may assume $S = \mathop{\mathrm{Spec}}(A)$ where $A$ is a reduced ring and $\eta $ corresponds to a minimal prime ideal $\mathfrak p$. Recall that the local ring $\mathcal{O}_{S, \eta } = A_\mathfrak p$ is equal to $\kappa (\mathfrak p)$ in this case, see Algebra, Lemma 10.25.1.
Apply Varieties, Lemma 33.25.11 to the scheme $X_\eta $ over $k = \kappa (\eta )$. Denote $k'/k$ the purely inseparable field extension this produces. In the next paragraph we reduce to the case $k' = k$. (This step corresponds to finding the morphism $V \to U$ in the statement of the lemma; in particular we can take $V = U$ if the characteristic of $\kappa (\mathfrak p)$ is zero.)
If the characteristic of $k = \kappa (\mathfrak p)$ is zero, then $k' = k$. If the characteristic of $k = \kappa (\mathfrak p)$ is $p > 0$, then $p$ maps to zero in $A_\mathfrak p = \kappa (\mathfrak p)$. Hence after replacing $A$ by a principal localization (i.e., shrinking $S$) we may assume $p = 0$ in $A$. If $k' \not= k$, then there exists an $\beta \in k'$, $\beta \not\in k$ such that $\beta ^ p \in k$. After replacing $A$ by a principal localization we may assume there exists an $a \in A$ such that $\beta ^ p = a$. Set $A' = A[x]/(x^ p - a)$. Then $S' = \mathop{\mathrm{Spec}}(A') \to \mathop{\mathrm{Spec}}(A) = S$ is finite locally free, surjective, and universally injective. Moreover, if $\mathfrak p' \subset A'$ denotes the unique prime ideal lying over $\mathfrak p$, then $A'_{\mathfrak p'} = k(\beta )$ and $k'/k(\beta )$ has smaller degree. Thus after replacing $S$ by $S'$ and $\eta $ by the point $\eta '$ corresponding to $\mathfrak p'$ we see that the degree of $k'$ over the residue field of $\eta $ has decreased. Continuing like this, by induction we reduce to the case $k' = \kappa (\mathfrak p) = \kappa (\eta )$.
Thus we may assume $S$ is affine, reduced, and that we have a $t \geq 0$ and closed subschemes
\[ X_\eta \supset Z_{\eta , 0} \supset Z_{\eta , 1} \supset \ldots \supset Z_{\eta , t} = \emptyset \]
such that $Z_{\eta , 0} = (X_\eta )_{red}$ and $Z_{\eta , i} \setminus Z_{\eta , i + 1}$ is smooth over $\eta $ for all $i$. Recall that $\kappa (\eta ) = \kappa (\mathfrak p) = A_\mathfrak p$ is the filtered colimit of $A_ a$ for $a \in A$, $a \not\in \mathfrak p$. See Algebra, Lemma 10.9.9. Thus we can descend the diagram above to a corresponding diagram over $\mathop{\mathrm{Spec}}(A_ a)$ for some $a \in A$, $a \not\in \mathfrak p$. More precisely, after replacing $S$ by $\mathop{\mathrm{Spec}}(A_ a)$ we may assume we have a $t \geq 0$ and closed subschemes
\[ X \supset Z_0 \supset Z_1 \supset \ldots \supset Z_ t = \emptyset \]
such that $Z_ i \to X$ is a closed immersion of finite presentation, such that $Z_0 \to X$ is a thickening, and such that $Z_ i \setminus Z_{i + 1}$ is smooth over $S$. In other words, the lemma holds. More precisely, we first use Limits, Lemma 32.10.1 to obtain morphisms
\[ Z_ t \to Z_{t - 1} \to \ldots \to Z_0 \to X \]
over $S$, each of finite presentation, and whose base change to $\eta $ produces the inclusions between the given closed subschemes above. After shrinking $S$ further we may assume each of the morphisms is a closed immersion, see Limits, Lemma 32.8.5. After shrinking $S$ we may assume $Z_0 \to X$ is surjective and hence a thickening, see Limits, Lemma 32.8.15. After shrinking $S$ once more we may assume $Z_ i \setminus Z_{i + 1} \to S$ is smooth, see Limits, Lemma 32.8.9. This finishes the proof.
$\square$
Lemma 37.55.2. Let $f : X \to S$ be a morphism of finite presentation between quasi-compact and quasi-separated schemes. Then there exists a $t \geq 0$ and closed subschemes
\[ S \supset S_0 \supset S_1 \supset \ldots \supset S_ t = \emptyset \]
such that
$S_ i \to S$ is defined by a finite type ideal sheaf,
$S_0 \subset S$ is a thickening,
for each $i$ there exists a surjective finite locally free morphism $T_ i \to S_ i \setminus S_{i + 1}$,
for each $i$ there exists a $t_ i \geq 0$ and closed subschemes
\[ X_ i = X \times _ S T_ i \supset Z_{i, 0} \supset Z_{i, 1} \supset \ldots \supset Z_{i, t_ i} = \emptyset \]
such that $Z_{i, j} \to X_ i$ is defined by a finite type ideal sheaf, $Z_{i, 0} \subset X_ i$ is a thickening, and such that the morphism $Z_{i, j} \setminus Z_{i, j + 1} \to T_ i$ is smooth.
Proof.
We can find a cartesian diagram
\[ \xymatrix{ X \ar[d] \ar[r] & X_0 \ar[d] \\ S \ar[r] & S_0 } \]
such that $X_0$ and $S_0$ are of finite type over $\mathbf{Z}$. See Limits, Proposition 32.5.4 and Lemma 32.10.1. Thus we may assume $X$ and $S$ are of finite type over $\mathbf{Z}$. Namely, a solution of the problem posed by the lemma for $X_0 \to S_0$ will base change to a solution over $S$; details omitted.
Assume $X$ and $S$ are of finite type over $\mathbf{Z}$. In this case every quasi-coherent ideal is of finite type, hence we do not have to check the condition that $S_ i$ is cut out by a finite type ideal. Set $S_0 = S_{red}$ equal to the reduction of $S$. Let $\eta \in S_0$ be a generic point of an irreducible component. By Lemma 37.55.1 we can find an open subscheme $U \subset S_0$, a surjective, universally injective, finite locally free morphism $V \to U$, a $t_0 \geq 0$ and closed subschemes
\[ X \times _ S V \supset Z_{0, 0} \supset Z_{0, 1} \supset \ldots \supset Z_{0, t_0} = \emptyset \]
such that $Z_{0, i} \to X \times _ S V$ is defined by a finite type ideal sheaf, $Z_{0, 0} \subset X \times _ S V$ is a thickening, and such that the morphism $Z_{0, i} \setminus Z_{0, i + 1} \to V$ is smooth. Then we let $S_1 \subset S_0$ be the reduced induced subscheme structure on $S_0 \setminus U$. By Noetherian induction on the underlying topological space of $S$, we may assume that the lemma holds for $X \times _ S S_1 \to S_1$. This produces $t \geq 1$ and
\[ S_1 = S_1 \supset S_2 \supset \ldots \supset S_ t = \emptyset \]
and $t_ i$ and $Z_{i, j}$ as in the statement of the lemma. This proves the lemma.
$\square$
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