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An Eilenberg–Mac Lane space is a connected topological space with nontrivial homotopy groups only in a single degree.
For and a group, and an abelian group if , then an Eilenberg-MacLane space is a topological space with the property that all its homotopy groups are trivial, except that in degree , which is .
For a group, the Eilenberg–Mac Lane space is the image under the homotopy hypothesis Quillen equivalence of the one-object groupoid whose hom-set is :
The construction of EM-spaces for an abelian group may be given by the Dold-Kan correspondence between chain complexes and simplicial abelian groups: let be the chain complex which is in dimension and zero elsewhere; the geometric realisation of the corresponding simplicial abelian group is then a .
We can include the case when may be nonabelian, by regarding as a crossed complex. Its classifying space is then a . (This also includes the case when is just a set!) This method also allows for the construction of where is a group, or groupoid, and is a -module. This gives a space with , all other homotopy trivial, and with the given operation of on .
For an abelian group, the Eilenberg–Mac Lane space is the image of the ∞-groupoid that is the strict ∞-groupoid given by the crossed complex that is trivial everywhere except in degree , where it is :
So
Therefore Eilenberg–Mac Lane spaces constitute a spectrum: the Eilenberg-Mac Lane spectrum.
In general, if is an abelian topological group, then there exist a model for the classifying space which is an abelian topological group. Iterating this construction, one has a notion of and a model for it which is an abelian topological group. If moreover is discrete, then , and one inductively sees that . Therefore one has a model for which is an abelian topological group.
See for instance (May, chapter 16, section 5)
For an abelian group and , the reduced -linearization of the n-sphere is the topological space, whose underlying set is the quotient
of the tensor product with of the free abelian group on the underlying set of , by the relation that identifies every formal linear combination of the (any fixed) basepoint of with 0. The topology is the induced quotient topology (of the disjoint union of product topological spaces, where is equipped with the discrete topology).
(Aguilar-Gitler-Prieto 02, def. 6.4.20)
For a countable abelian group, then the reduced -linearization (def. ) is an Eilenberg-MacLane space, in that its homotopy groups are
(in particular for then there is a unique connected component and hence we need not specify a basepoint for the homotopy group).
(Aguilar-Gitler-Prieto 02, corollary 6.4.23)
The topological space in definition has a canonical continuous action of the orthogonal group , by regarding as the one-point compactification of Cartesian space. Hence, in view of prop. , these models for EM-spaces lend themselves to the defintiion of Eilenberg-MacLane spectra as orthogonal spectra. See there.
One common use of Eilenberg–Mac Lane spaces is as coefficient objects for “ordinary” cohomology (see e.g. May, chapter 22).
The th “ordinary” cohomology of a topological space with coefficients in (when ) or (generally) is the collection of homotopy classes of maps from into or , respectively:
Here on the right and denotes the homotopy category of the (∞,1)-categories of topological spaces and of ∞-groupoids, respectively.
Not only the set is related to the cohomology of with coefficients in , but also the higher homotopy groups are, and in the most obvious way: if is a connected CW-complex, then
for any choice of base point on the right hand sides. This fact, which appears to have first been remarked by Thom and Federer, is an immediate consequence of the natural homotopy equivalences
and
one has in every -topos, see loop space object. For a nonabelian group, Gottlieb proves the following nonabelian analogue of the above result: let be a finite dimensional connected CW-complex; for a fixed map , let be the centralizer in of . Then the connected component of in is a .
Notice that for a nonabelian group, is a simple (and the most familiar) example of nonabelian cohomology. Nonabelian cohomology in higher degrees is obtained by replacing here the coefficient -groupoids of the simple form with more general -groupoids.
On the other hand there is the cohomology of Eilenberg-MacLane spaces itself. This is in general rich. Classical results by Serre and Henri Cartan are reviewed in (Clement 02, section 2).
For all even , the ordinary cohomology ring of with coefficients in the rational numbers is the polynomial algebra on the generator . For all odd it is the exterior algebra on this generator:
This is reviewed for instance in (Yin, section 4).
In p.130 of Eilenberg & MacLane (1954), some low-degree cohomology groups of are computed.
There are stable isomorphisms
for and .
There exists an isomorphism
where is the group of quadratic functions from valued in .
The notion of Eilenberg?Mac Lane object makes sense in every -topos, not just in Top. See at Eilenberg-MacLane object.
Eilenberg-MacLane spaces originate with:
Samuel Eilenberg, Saunders Mac Lane, On the Groups , I, Annals of Mathematics Second Series, Vol. 58, No. 1 (Jul., 1953), pp. 55-106 (jstor:1969820)
Samuel Eilenberg, Saunders Mac Lane, On the Groups , II: Methods of Computation, Annals of Mathematics Second Series, Vol. 60, No. 1 (Jul., 1954), pp. 49-139 (jstor:1969702)
Samuel Eilenberg, Saunders Mac Lane, On the Groups , III: Operations and Obstructions, Annals of Mathematics Second Series, Vol. 60, No. 3 (Nov., 1954), pp. 513-557 (jstor:1969849)
That Eilenberg-MacLane spaces represent ordinary cohomology is due to:
Samuel Eilenberg, p. 243 of: Cohomology and Continuous Mappings, Annals of Mathematics Second Series, Vol. 41, No. 1 (Jan., 1940), pp. 231-251 (jstor:1968828)
Early review is in
Textbook accounts:
Marcelo Aguilar, Samuel Gitler, Carlos Prieto, Algebraic topology from a homotopical viewpoint, Springer (2002) (toc pdf, doi:10.1007/b97586)
(EM-spaces are constructed in section 6, the cohomology theory they represent is discussed in section 7.1, and its equivalence to singular cohomology is Corollary 12.1.20)
Peter May, Chapter 22 of: A concise course in algebraic topology (pdf)
Lecture notes:
The construction via reduced linearization of spheres is considered in
Quick review includes
Formalization of Eilenberg-MacLane spaces in homotopy type theory (cf. Eilenberg-MacLane space type):
Dan Licata, Eric Finster, Eilenberg-MacLane spaces in homotopy type theory, CSL-LICS ‘14 66 (2014) 1-9 [pdf text, Agda HoTT code, web discussion, doi:10.1145/2603088.2603153]
Floris van Doorn, §42 in: On the Formalization of Higher Inductive Types and Synthetic Homotopy Theory (2018) [arXiv:1808.10690]
Ulrik Buchholtz, J. Daniel Christensen, Jarl G. Taxerås Flaten, Egbert Rijke, Central H-spaces and banded types [arXiv:2301.02636]
David Wärn, Eilenberg-MacLane spaces and stabilisation in homotopy type theory, J. Homotopy Relat. Struct. 18 (2023) 357–368 [arXiv:2301.03685, doi:10.1007/s40062-023-00330-5]
The ordinary cohomology of Eilenberg-MacLane spaces is discussed in
The topological K-theory of EM-spaces is discussed in
Last revised on January 31, 2024 at 09:26:37. See the history of this page for a list of all contributions to it.