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Leray spectral sequence
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In mathematics, the Leray spectral sequence was a pioneering example in homological algebra, introduced in 1946[1][2] by Jean Leray. It is usually seen nowadays as a special case of the Grothendieck spectral sequence.
Contents
1 Definition
2 Classical definition
3 Examples
4 Degeneration Theorem
4.1 Example with Monodromy
5 History and connection to other spectral sequences
Definition
Let f:X→Y be a continuous map of topological spaces, which in particular gives a functor f* from sheaves on X to sheaves on Y. Composing this with the functor Γ of taking sections on Sh(Y) is the same as taking sections on Sh(X), by the definition of the direct image functor f*:
History and connection to other spectral sequences
At the time of Leray's work, neither of the two concepts involved (spectral sequence, sheaf cohomology) had reached anything like a definitive state. Therefore it is rarely the case that Leray's result is quoted in its original form.
After much work, in the seminar of Henri Cartan in particular, the modern statement was obtained, though not the general Grothendieck spectral sequence.
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