Dold–Kan correspondence

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In mathematics, more precisely, in the theory of simplicial sets, the Dold–Kan correspondence (named after Albrecht Dold and Daniel Kan) states [1] that there is an equivalence between the category of (nonnegatively graded) chain complexes and the category of simplicial abelian groups. Moreover, under the equivalence, the th homology group of a chain complex is the th homotopy group of the corresponding simplicial abelian group, and a chain homotopy corresponds to a simplicial homotopy. (In fact, the correspondence preserves the respective standard model structures.) The correspondence is an example of the nerve and realization paradigm.

Contents

There is also an ∞-category-version of the Dold–Kan correspondence. [2]

The book "Nonabelian Algebraic Topology" cited below has a Section 14.8 on cubical versions of the Dold–Kan theorem, and relates them to a previous equivalence of categories between cubical omega-groupoids and crossed complexes, which is fundamental to the work of that book.

Examples

For a chain complex C that has an abelian group A in degree n and zero in all other degrees, the corresponding simplicial group is the Eilenberg–MacLane space .

Detailed construction

The Dold-Kan correspondence between the category sAb of simplicial abelian groups and the category Ch≥0(Ab) of nonnegatively graded chain complexes can be constructed explicitly through a pair of functors [1] pg 149 so that these functors form an equivalence of categories. The first functor is the normalized chain complex functor

and the second functor is the "simplicialization" functor

constructing a simplicial abelian group from a chain complex. The formed equivalence is an instance of a special type of adjunction, called the nerve-realization paradigm [3] (also called a nerve-realization context) where the data of this adjunction is determined by what's called a cosimplicial object , and the adjunction then takes the form

where we take the left Kan extension and is the Yoneda embedding.

Normalized chain complex

Given a simplicial abelian group there is a chain complex called the normalized chain complex (also called the Moore complex) with terms

and differentials given by

These differentials are well defined because of the simplicial identity

showing the image of is in the kernel of each . This is because the definition of gives . Now, composing these differentials gives a commutative diagram

and the composition map . This composition is the zero map because of the simplicial identity

and the inclusion , hence the normalized chain complex is a chain complex in . Because a simplicial abelian group is a functor

and morphisms are given by natural transformations, meaning the maps of the simplicial identities still hold, the normalized chain complex construction is functorial.

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References

  1. 1 2 Goerss & Jardine (1999) , Ch 3. Corollary 2.3
  2. Lurie , § 1.2.4.
  3. Loregian, Fosco (21 May 2023). Coend calculus. p. 85. Retrieved 26 November 2024.

Further reading