Lemma 54.3.1. Let $(A, \mathfrak m, \kappa )$ be a regular local ring of dimension $2$. Let $f : X \to S = \mathop{\mathrm{Spec}}(A)$ be the blowing up of $A$ in $\mathfrak m$ wotj exceptional divisor $E$. There is a closed immersion

\[ r : X \longrightarrow \mathbf{P}^1_ S \]

over $S$ such that

$r|_ E : E \to \mathbf{P}^1_\kappa $ is an isomorphism,

$\mathcal{O}_ X(E) = \mathcal{O}_ X(-1) = r^*\mathcal{O}_{\mathbf{P}^1}(-1)$, and

$\mathcal{C}_{E/X} = (r|_ E)^*\mathcal{O}_{\mathbf{P}^1}(1)$ and $\mathcal{N}_{E/X} = (r|_ E)^*\mathcal{O}_{\mathbf{P}^1}(-1)$.

**Proof.**
As $A$ is regular of dimension $2$ we can write $\mathfrak m = (x, y)$. Then $x$ and $y$ placed in degree $1$ generate the Rees algebra $\bigoplus _{n \geq 0} \mathfrak m^ n$ over $A$. Recall that $X = \text{Proj}(\bigoplus _{n \geq 0} \mathfrak m^ n)$, see Divisors, Lemma 31.32.2. Thus the surjection

\[ A[T_0, T_1] \longrightarrow \bigoplus \nolimits _{n \geq 0} \mathfrak m^ n, \quad T_0 \mapsto x,\ T_1 \mapsto y \]

of graded $A$-algebras induces a closed immersion $r : X \to \mathbf{P}^1_ S = \text{Proj}(A[T_0, T_1])$ such that $\mathcal{O}_ X(1) = r^*\mathcal{O}_{\mathbf{P}^1_ S}(1)$, see Constructions, Lemma 27.11.5. This proves (2) because $\mathcal{O}_ X(E) = \mathcal{O}_ X(-1)$ by Divisors, Lemma 31.32.4.

To prove (1) note that

\[ \left(\bigoplus \nolimits _{n \geq 0} \mathfrak m^ n\right) \otimes _ A \kappa = \bigoplus \nolimits _{n \geq 0} \mathfrak m^ n/\mathfrak m^{n + 1} \cong \kappa [\overline{x}, \overline{y}] \]

a polynomial algebra, see Algebra, Lemma 10.106.1. This proves that the fibre of $X \to S$ over $\mathop{\mathrm{Spec}}(\kappa )$ is equal to $\text{Proj}(\kappa [\overline{x}, \overline{y}]) = \mathbf{P}^1_\kappa $, see Constructions, Lemma 27.11.6. Recall that $E$ is the closed subscheme of $X$ defined by $\mathfrak m\mathcal{O}_ X$, i.e., $E = X_\kappa $. By our choice of the morphism $r$ we see that $r|_ E$ in fact produces the identification of $E = X_\kappa $ with the special fibre of $\mathbf{P}^1_ S \to S$.

Part (3) follows from (1) and (2) and Divisors, Lemma 31.14.2.
$\square$

## Comments (3)

Comment #4246 by Dario Weißmann on

Comment #4421 by Johan on

Comment #4423 by Dario Weißmann on