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单词 Stalk
释义

stalk


Let F be a presheafPlanetmathPlanetmathPlanetmath over a topological spaceMathworldPlanetmath X with values in an abelian categoryMathworldPlanetmathPlanetmathPlanetmath 𝒜, and suppose direct limitsMathworldPlanetmath exist in 𝒜. For any point pX, the stalk Fp of F at p is defined to be the object in 𝒜 which is the direct limit of the objects F(U) over the directed set of all open sets UX containing p, with respect to the restrictionPlanetmathPlanetmathPlanetmath morphismsMathworldPlanetmathPlanetmath of F. In other words,

Fp:=limUpF(U)

If 𝒜 is a category consisting of sets, the stalk Fp can be viewed as the set of all germs of sectionsPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath of F at the point p. That is, the set Fp consists of all the equivalence classesMathworldPlanetmathPlanetmath of ordered pairsMathworldPlanetmath (U,s) where pU and sF(U), under the equivalence relation (U,s)(V,t) if there exists a neighborhoodMathworldPlanetmathPlanetmath WUV of p such that resU,Ws=resV,Wt.

By universal propertiesMathworldPlanetmath of direct limit, a morphism ϕ:FG of presheaves over X induces a morphism ϕp:FpGp on each stalk Fp of F. Stalks are most useful in the context of sheaves, since they encapsulate all of the local data of the sheaf at the point p (recall that sheaves are basically defined as presheaves which have the property of being completely characterized by their local behavior). Indeed, in many of the standard examples of sheaves that take values in rings (such as the sheaf 𝒟X of smooth functions, or the sheaf 𝒪X of regular functions), the ring Fp is a local ring, and much of geometry is devoted to the study of sheaves whose stalks are local rings (so-called “locally ringed spaces”).

We mention here a few illustrations of how stalks accurately reflect the local behavior of a sheaf; all of these are drawn from [1].

  • A morphism of sheaves ϕ:FG over X is an isomorphismMathworldPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath if and only if the induced morphism ϕp is an isomorphism on each stalk.

  • A sequencePlanetmathPlanetmath FGH of morphisms of sheaves over X is an exact sequencePlanetmathPlanetmathPlanetmath at G if and only if the induced morphism FpGpHp is exact at each stalk Gp.

  • The sheafificationPlanetmathPlanetmath F of a presheaf F has stalk equal to Fp at every point p.

References

  • 1 Robin Hartshorne, Algebraic GeometryMathworldPlanetmathPlanetmath, Springer–Verlag New York Inc., 1977 (GTM 52).
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更新时间:2025/5/4 5:41:03