HochschildWitt homology

General Introduction
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Search results
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Online References
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Paper References
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Definition
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Properties
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Standard theorems
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Open Problems
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Connections to Number Theory
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Computations and Examples
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History and Applications
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Some Research Articles
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Other Information
Dmitry Kaledin (Steklov Institute): The HochschildWitt complex
Witt vectors were introduced by E. Witt back in 1930ies. In modern language, Witt's construction gives a funcntorial way to lift a commutative ring A in char p to a commutative ring W(A) of characteristic 0. Since the construction is functorial, it can be applied to the structure sheaf of an algebraic variety. However, this really became useful only in the 1970ies, when Deligne and Illusie showed how to extend it to differential forms. The result is a canonical "de RhamWitt complex" WO* on any smooth algebraic variety X over a field of char p, and the cohomology of X with coefficients in WO* is canonically identified with Grothendieck's cristalline cohomology of X. I am going to report on some recent discoveries which can be roughly summarized as follows: even for a noncommutative ring A, one can define a functorial "HochschildWitt complex" with homology WHH*(A); if A is commutative, then WHHi(A)=WO^i(A) (this is analogous to the isomorphism HH_i(A)=O^i(A) discovered by Hochschild, Kostant and Rosenberg). Moreover, the construction of the HochschildWitt complex is actually simpler than the DeligneIllusie construction, and it allows to clarify the structure of the de RhamWitt complex. I will start very slowly, by introducing Witt vectors in the simplest possible case; at least in the first lecture, there will be definitely nothing noncommutative. I will assume very little prior knowledge. In particular, I will not assume knowledge of either Witt vectors or cristalline cohomology, and I will explain all the necessary material along the way.
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