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Questions designed to get an overview of a specific subject or body of results or to understand the relations among similar definitions, techniques or concepts appearing in different sub-fields of mathematics. While such questions by their very nature sometimes cannot be made very narrow and focused, it can be helpful to keep in mind that the design of MathOverflow does not make it a good fit for questions that are too broad.
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How should one think about sheafification and the difference between a sheaf and a presheaf
There are two ways a presheaf can fail to be a sheaf.
It has local sections that should patch together to give a global section, but don't,
It has non-zero sections which are locally zero.
When di …
- 13k
55
votes
Describe a topic in one sentence.
Homological algebra - In an abelian category, the difference between what you wish was true and what IS true is measured by a homology group.