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topos theory

Contents

Definition

Given a sheaf $F$ of sets over a topological space $X$, the section over an arbitrary (= not necessarily open) subset $V\subset X$ is a continuous section of the corresponding etale space restricted to $V$.

A sheaf $F$ of sets (or of abelian groups) over a paracompact Hausdorff space $X$ is soft if for any closed subset $V\subset X$, every section of $F$ over $V$ can be extended to the whole $X$.

Properties

The local extension of germs to the open neighborhoods of points by paracompactness gives rise to an extension of the section to an open neighborhood of the whole set $V$. Therefore every flabby sheaf is soft, because flabbiness gives the extension from open subsets. Fine sheaves are always soft.

References

Standard references are Tohoku and

• Roger Godement, Topologie Algébrique et Théorie des Faisceaux. Actualités Sci. Ind. No. 1252. Publ. Math. Univ. Strasbourg. No. 13 Hermann, Paris 1958.
category: sheaf theory

Last revised on April 1, 2019 at 06:01:32. See the history of this page for a list of all contributions to it.