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Sheaf


A sheaf is a presheaf with "something" added allowing us to define things locally. This task is forbidden for presheaves in general. Specifically, a presheaf F on a topological space X is a sheaf if it satisfies the following conditions:

1. if U is an open set, if {U_i} is an open covering of U and if s in F(U) is an element such that s|_(U_i)=0 for all i, then s=0.

2. if U is an open set, if {U_i} is an open covering of U and if we have elements s_i in F(U_i) for each i, with the property that for each, i,j, s_i|_(U_i intersection U_j)=s_j|_(U_i intersection U_j), then there is an element s in F(U) such that s|_(U_i)=s_i for all i.

The first condition implies that s is unique.

For example, let X be a variety over a field k. If O(U) denotes the ring of regular functions from U to k then with the usual restrictions O is a sheaf which is called the sheaf of regular functions on X.

In the same way, one can define the sheaf of continuous real-valued functions on any topological space, and also for differentiable functions.


See also

Perverse Sheaf, Presheaf, Sheaf of Planes, Star, Topological Sheaf

This entry contributed by José Gallardo Alberni

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References

Godement, R. Topologie Algébrique et Théorie des Faisceaux. Paris: Hermann, 1958.Hartshorne, R. Algebraic Geometry. New York: Springer-Verlag, 1977.Iyanaga, S. and Kawada, Y. (Eds.). "Sheaves." §377 in Encyclopedic Dictionary of Mathematics. Cambridge, MA: MIT Press, pp. 1171-1174, 1980.

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Sheaf

Cite this as:

Alberni, José Gallardo. "Sheaf." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/Sheaf.html

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