Quasi-finite morphism

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In algebraic geometry, a branch of mathematics, a morphism f : XY of schemes is quasi-finite if it is finite type and satisfies any of the following equivalent conditions:[1]

  • Every point x of X is isolated in its fiber f−1(f(x)). In other words, every fiber is a discrete (hence finite) set.
  • For every point x of X, the scheme f−1(f(x)) = X ×YSpec κ(f(x)) is a finite κ(f(x)) scheme. (Here κ(p) is the residue field at a point p.)
  • For every point x of X, \mathcal{O}_{X,x}\otimes \kappa(f(x)) is finitely generated over \kappa(f(x)).

Quasi-finite morphisms were originally defined by Alexander Grothendieck in SGA 1 and did not include the finite type hypothesis. This hypothesis was added to the definition in EGA II 6.2 because it makes it possible to give an algebraic characterization of quasi-finiteness in terms of stalks.

For a general morphism f : XY and a point x in X, f is said to be quasi-finite at x if there exist open affine neighborhoods U of x and V of f(x) such that f(U) is contained in V and such that the restriction f : UV is quasi-finite. f is locally quasi-finite if it is quasi-finite at every point in X.[2] A quasi-compact locally quasi-finite morphism is quasi-finite.

Properties[edit]

For a morphism f, the following properties are true.[3]

  • If f is quasi-finite, then the induced map fred between reduced schemes is quasi-finite.
  • If f is a closed immersion, then f is quasi-finite.
  • If X is noetherian and f is an immersion, then f is quasi-finite.
  • If g : YZ, and if gf is quasi-finite, then f is quasi-finite if any of the following are true:
    1. g is separated,
    2. X is noetherian,
    3. X ×Z Y is locally noetherian.

Quasi-finiteness is preserved by base change. The composite and fiber product of quasi-finite morphisms is quasi-finite.[3]

If f is unramified at a point x, then f is quasi-finite at x. Conversely, if f is quasi-finite at x, and if also \mathcal{O}_{f^{-1}(f(x)),x}, the local ring of x in the fiber f−1(f(x)), is a field and a finite separable extension of κ(f(x)), then f is unramified at x.[4]

Finite morphisms are quasi-finite.[5] A quasi-finite proper morphism locally of finite presentation is finite.[6]

A generalized form of Zariski Main Theorem is the following:[7] Suppose Y is quasi-compact and quasi-separated. Let f be quasi-finite, separated and of finite presentation. Then f factors as  X \hookrightarrow X' \to Y where the first morphism is an open immersion and the second is finite. (X is open in a finite scheme over Y.)

Notes[edit]

  1. ^ EGA II, Définition 6.2.3
  2. ^ EGA III, ErrIII, 20.
  3. ^ a b EGA II, Proposition 6.2.4.
  4. ^ EGA IV4, Théorème 17.4.1.
  5. ^ EGA II, Corollaire 6.1.7.
  6. ^ EGA IV3, Théorème 8.11.1.
  7. ^ EGA IV3, Théorème 8.12.6.

References[edit]