Morphisms of schemes

Flat morphism

In mathematics, in particular in the theory of schemes in algebraic geometry, a flat morphism f from a scheme X to a scheme Y is a morphism such that the induced map on every stalk is a flat map of rings, i.e., is a flat map for all P in X. A map of rings is called flat if it is a homomorphism that makes B a flat A-module. A morphism of schemes is called faithfully flat if it is both surjective and flat. Two basic intuitions regarding flat morphisms are: * flatness is a generic property; and * the failure of flatness occurs on the jumping set of the morphism. The first of these comes from commutative algebra: subject to some on f, it can be shown that there is a non-empty open subscheme of Y, such that f restricted to Y′ is a flat morphism (generic flatness). Here 'restriction' is interpreted by means of the fiber product of schemes, applied to f and the inclusion map of into Y. For the second, the idea is that morphisms in algebraic geometry can exhibit discontinuities of a kind that are detected by flatness. For instance, the operation of blowing down in the birational geometry of an algebraic surface, can give a single fiber that is of dimension 1 when all the others have dimension 0. It turns out (retrospectively) that flatness in morphisms is directly related to controlling this sort of semicontinuity, or one-sided jumping. Flat morphisms are used to define (more than one version of) the flat topos, and flat cohomology of sheaves from it. This is a deep-lying theory, and has not been found easy to handle. The concept of étale morphism (and so étale cohomology) depends on the flat morphism concept: an étale morphism being flat, of finite type, and unramified. (Wikipedia).

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Related pages

Birational geometry | Stalk (sheaf) | Zero divisor | Algebraic surface | Commutative algebra | Degeneration (algebraic geometry) | Cohen–Macaulay ring | Inclusion map | Relative effective Cartier divisor | Fiber product of schemes | Flat module | Mathematics | Ramification (mathematics) | Elliptic surface | Algebraic geometry | Blowing up | Generic property | Scheme (mathematics) | Hilbert scheme | Fpqc morphism | Étale morphism | Étale cohomology | Generic flatness