Lemma 31.21.9. Let $i : Z \to Y$ and $j : Y \to X$ be immersions of schemes. Pick $z \in Z$ and denote $y \in Y$, $x \in X$ the corresponding points. Assume $X$ is locally Noetherian. The following are equivalent

1. $i$ is a regular immersion in a neighbourhood of $z$ and $j$ is a regular immersion in a neighbourhood of $y$,

2. $i$ and $j \circ i$ are regular immersions in a neighbourhood of $z$,

3. $j \circ i$ is a regular immersion in a neighbourhood of $z$ and the conormal sequence

$0 \to i^*\mathcal{C}_{Y/X} \to \mathcal{C}_{Z/X} \to \mathcal{C}_{Z/Y} \to 0$

is split exact in a neighbourhood of $z$.

Proof. Since $X$ (and hence $Y$) is locally Noetherian all 4 types of regular immersions agree, and moreover we may check whether a morphism is a regular immersion on the level of local rings, see Lemma 31.20.8. The implication (1) $\Rightarrow$ (2) is Lemma 31.21.7. The implication (2) $\Rightarrow$ (3) is Lemma 31.21.6. Thus it suffices to prove that (3) implies (1).

Assume (3). Set $A = \mathcal{O}_{X, x}$. Denote $I \subset A$ the kernel of the surjective map $\mathcal{O}_{X, x} \to \mathcal{O}_{Y, y}$ and denote $J \subset A$ the kernel of the surjective map $\mathcal{O}_{X, x} \to \mathcal{O}_{Z, z}$. Note that any minimal sequence of elements generating $J$ in $A$ is a quasi-regular hence regular sequence, see Lemma 31.20.5. By assumption the conormal sequence

$0 \to I/IJ \to J/J^2 \to J/(I + J^2) \to 0$

is split exact as a sequence of $A/J$-modules. Hence we can pick a minimal system of generators $f_1, \ldots , f_ n, g_1, \ldots , g_ m$ of $J$ with $f_1, \ldots , f_ n \in I$ a minimal system of generators of $I$. As pointed out above $f_1, \ldots , f_ n, g_1, \ldots , g_ m$ is a regular sequence in $A$. It follows directly from the definition of a regular sequence that $f_1, \ldots , f_ n$ is a regular sequence in $A$ and $\overline{g}_1, \ldots , \overline{g}_ m$ is a regular sequence in $A/I$. Thus $j$ is a regular immersion at $y$ and $i$ is a regular immersion at $z$. $\square$

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