In mathematics, an Eilenberg–MacLane space[note 1] is a special kind of topological space that can be regarded as a building block for homotopy theory. These spaces are important in many contexts in algebraic topology, including constructions of spaces, computations of homotopy groups of spheres, and definition of cohomology operations. The name is for Samuel Eilenberg and Saunders Mac Lane, who introduced such spaces in the late 1940s.
Let G be a group and n a positive integer. A connected topological space X is called an Eilenberg–MacLane space of type K(G, n), if it has n-th homotopy group πn(X) isomorphic to G and all other homotopy groups trivial. If n > 1 then G must be abelian. Then an Eilenberg–MacLane space exists, as a CW-complex, and is unique up to a weak homotopy equivalence. By abuse of language, any such space is often called just K(G, n).
- The unit circle S1 is a K(Z,1).
- The infinite-dimensional complex projective space P∞(C) is a model of K(Z,2). This is one of the rare examples of classifying spaces admitting a manifold model, and is also the topological space the homotopy groups of which satisfy πi = 0 for i = 1 and i > 2, while π2 = Z. Its cohomology ring is Z[x], namely the free polynomial ring on a single 2-dimensional generator x ∈ H2. The generator can be represented in de Rham cohomology by the Fubini–Study 2-form. An application of K(Z,2) is described at Abstract nonsense.
- The infinite-dimensional real projective space P∞(R) is a K(Z2, 1).
- The wedge sum of k unit circles is a K(G, 1) for G the free group on k generators.
- The complement to any knot in a 3-dimensional sphere S3 is of type K(G, 1); this is called the "asphericity of knots", and is a 1957 theorem of Christos Papakyriakopoulos.
Some further elementary examples can be constructed from these by using the obvious fact that the product K(G, n) × K(H, n) is K(G × H, n).
A K(G, n) can be constructed stage-by-stage, as a CW complex, starting with a wedge of n-spheres, one for each generator of the group G, and adding cells in (possibly infinite number of) higher dimensions so as to kill all extra homotopy.
Properties of Eilenberg–MacLane spaces
An important property of K(G, n) is that, for any abelian group G, and any CW-complex X, the set
- [X, K(G, n)]
of homotopy classes of maps from X to K(G, n) is in natural bijection with the n-th singular cohomology group
- Hn(X; G)
of the space X. Thus one says that the K(G, n) are representing spaces for cohomology with coefficients in G. Since
there is a distinguished element corresponding to the identity. The above bijection is given by pullback of that element — .
Another version of this result, due to Peter J. Huber, establishes a bijection with the n-th Čech cohomology group when X is Hausdorff and paracompact and G is countable, or when X is Hausdorff, paracompact and compactly generated and G is arbitrary. A further result of Morita establishes a bijection with the n-th numerable Čech cohomology group for an arbitrary topological space X and G an arbitrary abelian group.
Every CW-complex possesses a Postnikov tower, that is, it is homotopy equivalent to an iterated fibration with fibers the Eilenberg–MacLane spaces.
There is a method due to Jean-Pierre Serre which allows one, at least theoretically, to compute homotopy groups of spaces using a spectral sequence for special fibrations with Eilenberg–MacLane spaces for fibers.
The cohomology groups of Eilenberg–MacLane spaces can be used to classify all cohomology operations.
- S. Eilenberg, S. MacLane, Relations between homology and homotopy groups of spaces Ann. of Math. 46 (1945) pp. 480–509
- S. Eilenberg, S. MacLane, Relations between homology and homotopy groups of spaces. II Ann. of Math. 51 (1950) pp. 514–533
- Peter J. Huber (1961), Homotopical cohomology and Čech cohomology, Mathematische Annalen 144 , 73–76.
- Morita, Kiiti (1975). "Čech cohomology and covering dimension for topological spaces". Fundamenta Mathematicae 87: 31–52.
- Papakyriakopoulos, C. D. (1957). "On Dehn's Lemma and the Asphericity of Knots". Proc. Nat. Acad. Sci. USA 43 (1): 169–172. doi:10.1073/pnas.43.1.169. PMC 528404. PMID 16589993.
- Papakyriakopoulos, C. D. (1957). "On Dehn's Lemma and the Asphericity of Knots". Ann. Math. 66 (1): 1–26. doi:10.2307/1970113. JSTOR 1970113.
- Rudyak, Yu.B. (2001), "Eilenberg−MacLane space", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4