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Bar complex
Technique for constructing resolutions in homological algebra From Wikipedia, the free encyclopedia
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In mathematics, the bar complex, also called the bar resolution, bar construction, standard resolution, or standard complex, is a way of constructing resolutions in homological algebra. It was first introduced for the special case of algebras over a commutative ring by Samuel Eilenberg and Saunders Mac Lane (1953) and Henri Cartan and Eilenberg (1956, IX.6) and has since been generalized in many ways. The name "bar complex" comes from the fact that Eilenberg & Mac Lane (1953) used a vertical bar | as a shortened form of the tensor product in their notation for the complex.
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Definition
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Let be an algebra over a field , let be a right -module, and let be a left -module. Then, one can form the bar complex given by
with the differential
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Resolutions
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The bar complex is useful because it provides a canonical way of producing (free) resolutions of modules over a ring. However, often these resolutions are very large, and can be prohibitively difficult to use for performing actual computations.
Free Resolution of a Module
Let be a left -module, with a unital -algebra. Then, the bar complex gives a resolution of by free left -modules. Explicitly, the complex is[1]
This complex is composed of free left -modules, since each subsequent term is obtained by taking the free left -module on the underlying vector space of the previous term.
To see that this gives a resolution of , consider the modified complex
Then, the above bar complex being a resolution of is equivalent to this extended complex having trivial homology. One can show this by constructing an explicit homotopy between the identity and 0. This homotopy is given by
One can similarly construct a resolution of a right -module by free right modules with the complex .
Notice that, in the case one wants to resolve as a module over itself, the above two complexes are the same, and actually give a resolution of by --bimodules. This provides one with a slightly smaller resolution of by free --bimodules than the naive option . Here we are using the equivalence between --bimodules and -modules, where , see bimodules for more details.
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The Normalized Bar Complex
The normalized (or reduced) standard complex replaces with .
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