Lemma 54.4.2. Let $X$ be a Noetherian scheme. Let $T \subset X$ be a finite set of closed points $x$ such that $\mathcal{O}_{X, x}$ is a regular local ring of dimension $2$. Let $f : Y \to X$ be a proper morphism of schemes which is an isomorphism over $U = X \setminus T$. Then there exists a sequence

$X_ n \to X_{n - 1} \to \ldots \to X_1 \to X_0 = X$

where $X_{i + 1} \to X_ i$ is the blowing up of $X_ i$ at a closed point $x_ i$ lying above a point of $T$ and a factorization $X_ n \to Y \to X$ of the composition.

Proof. By More on Flatness, Lemma 38.31.4 there exists a $U$-admissible blowup $X' \to X$ which dominates $Y \to X$. Hence we may assume there exists an ideal sheaf $\mathcal{I} \subset \mathcal{O}_ X$ such that $\mathcal{O}_ X/\mathcal{I}$ is supported on $T$ and such that $Y$ is the blowing up of $X$ in $\mathcal{I}$. By Lemma 54.4.1 there exists a sequence

$X_ n \to X_{n - 1} \to \ldots \to X_1 \to X_0 = X$

where $X_{i + 1} \to X_ i$ is the blowing up of $X_ i$ at a closed point $x_ i$ lying above a point of $T$ such that $\mathcal{I}\mathcal{O}_{X_ n}$ is an invertible ideal sheaf. By the universal property of blowing up (Divisors, Lemma 31.32.5) we find the desired factorization. $\square$

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