Lemma 29.48.6. Let f : Y \to X be a finite morphism with X affine. There exists a diagram
\xymatrix{ Z' \ar[rd] & Y' \ar[l]^ i \ar[d] \ar[r] & Y \ar[d] \\ & X' \ar[r] & X }
where
Y' \to Y and X' \to X are surjective finite locally free,
Y' = X' \times _ X Y,
i : Y' \to Z' is a closed immersion,
Z' \to X' is finite locally free, and
Z' = \bigcup _{j = 1, \ldots , m} Z'_ j is a (set theoretic) finite union of closed subschemes, each of which maps isomorphically to X'.
Proof.
Write X = \mathop{\mathrm{Spec}}(A) and Y = \mathop{\mathrm{Spec}}(B). See also More on Algebra, Section 15.21. Let x_1, \ldots , x_ n \in B be generators of B over A. For each i we can choose a monic polynomial P_ i(T) \in A[T] such that P(x_ i) = 0 in B. By Algebra, Lemma 10.136.14 (applied n times) there exists a finite locally free ring extension A \subset A' such that each P_ i splits completely:
P_ i(T) = \prod \nolimits _{k = 1, \ldots , d_ i} (T - \alpha _{ik})
for certain \alpha _{ik} \in A'. Set
C = A'[T_1, \ldots , T_ n]/(P_1(T_1), \ldots , P_ n(T_ n))
and B' = A' \otimes _ A B. The map C \to B', T_ i \mapsto 1 \otimes x_ i is an A'-algebra surjection. Setting X' = \mathop{\mathrm{Spec}}(A'), Y' = \mathop{\mathrm{Spec}}(B') and Z' = \mathop{\mathrm{Spec}}(C) we see that (1) – (4) hold. Part (5) holds because set theoretically \mathop{\mathrm{Spec}}(C) is the union of the closed subschemes cut out by the ideals
(T_1 - \alpha _{1k_1}, T_2 - \alpha _{2k_2}, \ldots , T_ n - \alpha _{nk_ n})
for any 1 \leq k_ i \leq d_ i.
\square
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