Lemma 10.126.11. Suppose $R \to S$ is a local homomorphism of local rings. Assume that $S$ is essentially of finite presentation over $R$. Then there exists a directed set $(\Lambda , \leq )$, and a system of local homomorphism $R_\lambda \to S_\lambda$ of local rings such that

1. The colimit of the system $R_\lambda \to S_\lambda$ is equal to $R \to S$.

2. Each $R_\lambda$ is essentially of finite type over $\mathbf{Z}$.

3. Each $S_\lambda$ is essentially of finite type over $R_\lambda$.

4. For each $\lambda \leq \mu$ the map $S_\lambda \otimes _{R_\lambda } R_\mu \to S_\mu$ presents $S_\mu$ as the localization of $S_\lambda \otimes _{R_\lambda } R_\mu$ at a prime ideal.

Proof. By assumption we may choose an isomorphism $\Phi : (R[x_1, \ldots , x_ n]/I)_{\mathfrak q} \to S$ where $I \subset R[x_1, \ldots , x_ n]$ is a finitely generated ideal, and $\mathfrak q \subset R[x_1, \ldots , x_ n]/I$ is a prime. (Note that $R \cap \mathfrak q$ is equal to the maximal ideal $\mathfrak m$ of $R$.) We also choose generators $f_1, \ldots , f_ m \in I$ for the ideal $I$. Write $R$ in any way as a colimit $R = \mathop{\mathrm{colim}}\nolimits R_\lambda$ over a directed set $(\Lambda , \leq )$, with each $R_\lambda$ local and essentially of finite type over $\mathbf{Z}$. There exists some $\lambda _0 \in \Lambda$ such that $f_ j$ is the image of some $f_{j, \lambda _0} \in R_{\lambda _0}[x_1, \ldots , x_ n]$. For all $\lambda \geq \lambda _0$ denote $f_{j, \lambda } \in R_{\lambda }[x_1, \ldots , x_ n]$ the image of $f_{j, \lambda _0}$. Thus we obtain a system of ring maps

$R_\lambda [x_1, \ldots , x_ n]/(f_{1, \lambda }, \ldots , f_{n, \lambda }) \to R[x_1, \ldots , x_ n]/(f_1, \ldots , f_ n) \to S$

Set $\mathfrak q_\lambda$ the inverse image of $\mathfrak q$. Set $S_\lambda = (R_\lambda [x_1, \ldots , x_ n]/ (f_{1, \lambda }, \ldots , f_{n, \lambda }))_{\mathfrak q_\lambda }$. We leave it to the reader to see that this works. $\square$

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