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In mathematics, particularly algebraic topology, the Kan-Thurston theorem associates a discrete group G to every path connected topological space X in such a way that the group cohomology of G is the same as the cohomology of the space X. The group G might then be regarded as a good approximation to the space X, and consequently the theorem is sometimes interpreted to mean that homotopy theory can be viewed as part of group theory.

More precisely[1], the theorem states that every path connected topological space is homology-equivalent to the classifying space \({\displaystyle K(G,1)} \) of a discrete group G, where homology-equivalent means there is a map \( {\displaystyle K(G,1)\rightarrow X} \) inducing an isomorphism on homology.

The theorem is attributed to Daniel Kan and William Thurston who published their result in 1976.
Statement of the Kan-Thurston theorem

Let X be a path connected topological space. Then, naturally associated to X, there is a Serre fibration \( {\displaystyle t_{x}\colon T_{X}\to X} \) where \( {\displaystyle T_{X}} \) is an aspherical space. Furthermore,

the induced map \( {\displaystyle \pi _{1}(T_{X})\to \pi _{1}(X)} \) is surjective, and
for every local coefficient system A on X, the maps \( {\displaystyle H_{*}(TX;A)\to H_{*}(X;A)} \) and \( {\displaystyle H^{*}(TX;A)\to H^{*}(X;A)} \) induced by t x {\displaystyle t_{x}} t_{x} are isomorphisms.


Kan-Thurston theorem in nLab

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Kim, Raeyong (2015). "Every finite complex has the homology of some CAT(0) cubical duality group". Geometriae Dedicata. 176: 1–9. doi:10.1007/s10711-014-9956-4. ISSN 0046-5755. MR 3347570.

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