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A topological 7-sphere equipped with an exotic smooth structure is called an exotic 7-sphere.
Milnor (1956) gave the first examples of exotic smooth structures on the 7-sphere, finding at least seven.
The exotic 7-spheres constructed in Milnor 1956 are all examples of fibre bundles over the 4-sphere with fibre the 3-sphere , with structure group the special orthogonal group SO(4) (see also at 8-manifold the section With exotic boundary 7-spheres):
By the classification of bundles on spheres via the clutching construction, these correspond to homotopy classes of maps , i.e. elements of . From the table at orthogonal group – Homotopy groups, this latter group is . Thus any such bundle can be described, up to isomorphism, by a pair of integers . When , then one can show there is a Morse function with exactly two critical points on the total space of the bundle, and hence this 7-manifold is homeomorphic to a sphere.
The fractional first Pontryagin class of the bundle is given by . Milnor constructs, using cobordism theory and Hirzebruch's signature theorem for 8-manifolds, a modulo-7 diffeomorphism invariant of the manifold, so that it is the standard 7-sphere precisely when .
By using the connected sum operation, the set of smooth, non-diffeomorphic structures on the -sphere has the structure of an abelian group. For the 7-sphere, it is the cyclic group and Brieskorn (1966) found the generator so that is the standard sphere.
Review includes (Kreck 10, chapter 19, McEnroe 15, Joachim-Wraith).
From the point of view of M-theory on 8-manifolds, these 8-manifolds with (exotic) 7-sphere boundaries in Milnor’s construction correspond to near horizon limits of black M2 brane spacetimes , where the M2-branes themselves would be sitting at the center of the 7-spheres (if that were included in the spacetime, see also Dirac charge quantization).
(Morrison-Plesser 99, section 3.2, FSS 19, 3.8))
John Milnor, On manifolds homeomorphic to the 7-sphere, Annals of Mathematics 64 (2): 399–405 (1956) (pdf, doi:10.1142/9789812836878_0001)
Egbert Brieskorn, Beispiele zur Differentialtopologie von Singularitäten, Inventiones mathematicae 2 (1966) 1–14 doi:10.1007/BF01403388
Matthias Kreck, chapter 19 of Exotic 7-spheres of Differential Algebraic Topology – From Stratifolds to Exotic Spheres, AMS 2010
Rachel McEnroe, Milnor’ construction of exotic 7-spheres, 2015 (pdf)
Michael Joachim, D. J. Wraith, Exotic spheres and curvature (pdf)
Niles Johnson, Visualizing 7-manifolds, 2012 (nilesjohnson.net/seven-manifolds.html)
Diarmuid Crowley, Christine Escher, A classification of -bundles over , Differential Geometry and its Applications Volume 18, Issue 3, May 2003, Pages 363-380 (doi:10.1016/S0926-2245(03)00012-3))
See also
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