- 5-manifold
In
mathematics , a 5-manifold is a 5-dimensionaltopological manifold , possibly with a piecewise linear or smooth structure.Non-simply connected 5-manifolds are impossible to classify, as this is harder than solving the
word problem for groups . Simply connected compact 5-manifolds were first classified byDennis Barden and another proof was later given by A. V. Zhubr. Rather surprisingly, this turns out to be easier than the 3- or 4-dimensional case: the 3-dimensional case is thePoincaré conjecture , and the 4-dimensional case was solved by Freedman (1982) in the topological case, but is a very hard unsolved problem in the smooth case.Indeed, in dimension 5 smooth classification is governed by classical algebraic topology, namely, two simply connected 5-manifolds are diffeomorphic if and only if there exists an isomorphism of their second homology groups with integer coefficients, preserving linking form and the second
Stiefel–Whitney class . Moreover any such isomorphism is induced by some diffeomorphism.References
*D. Barden, [http://links.jstor.org/sici?sici=0003-486X%28196511%292%3A82%3A3%3C365%3ASCF%3E2.0.CO%3B2-R "Simply Connected Five-Manifolds"] The Annals of Mathematics > 2nd Ser., Vol. 82, No. 3 (Nov., 1965), pp. 365-385
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