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    Locally of finite type, finite type, and finite presentation

    D-MORFT

    (Finite type)

    \(f : X \to Y\) is locally of finite type if \(Y\) has an affine cover by \(\Spec B_i\) such that \(f^{-1}(\Spec B_i)\) has an affine cover by \(\Spec A_{ij}\) with each \(A_{ij}\) a finitely generated \(B_i\)-algebra. It is of finite type if in addition each \(f^{-1}(\Spec B_i)\) is covered by finitely many of the \(\Spec A_{ij}\), that is, locally of finite type and quasicompact.

    (Finite presentation)

    \(f\) is locally of finite presentation if each \(B_i \to A_{ij}\) is of finite presentation: \(B_i[x_1, \dots, x_n] \surjects A_{ij}\) with finitely generated kernel. It is of finite presentation if it is also quasicompact and quasi-separated.

    Finite type is a hypothesis of the valuative criterion of properness and part of the definition of a proper morphism. Over a locally Noetherian base \(Y\), locally of finite type and locally of finite presentation coincide, because every ideal of \(B[x_1, \dots, x_n]\) is finitely generated for \(B\) Noetherian; for \(Y\) Noetherian, finite type and finite presentation coincide. Over the non-Noetherian ring \(B=k[x_1,x_2,\ldots]\), the quotient \(B\to B/(x_1,x_2,\ldots)\cong k\) is of finite type and not of finite presentation. Finite presentation is the finiteness condition in the definition of smooth and étale morphisms and in limit arguments.

    What is a morphism locally of finite type?
    What is a finite type morphism?
    What is a finitely presented morphism, and when does it differ from finite type?

    Backlinks

    • D-MORFINFinite morphisms, and finite against finite typerelated-to
    • D-MORUNRUnramified morphismsrelated-to
    • D-MORSMSmooth morphisms and relative dimensionuses

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