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Berger's sphere : ウィキペディア英語版 | Berger's sphere In Riemannian geometry, a Berger sphere, named after Marcel Berger, is a standard 3-sphere with Riemannian metric from a one-parameter family, which can be obtained from the standard metric by shrinking along fibers of a Hopf fibration. It is interesting in that it is one of the simplest examples of Gromov collapse.〔. See in particular (p. 122 ).〕 More precisely, one first considers the Lie algebra spanned by generators ''x''1, ''x''2, ''x''3 with Lie bracket () = −2ε''ijk''''x''''k''. This is well known to correspond to the simply connected Lie group ''S''3. Then, taking the product ''S''3×R, extending the Lie bracket so that the generator ''x''4 is left invariant under the operation of the Lie group, and taking the quotient by α''x''1+β''x''4, where α2+β2 = 1, we finally obtain the Berger spheres ''B''(β).〔.〕 There are also higher-dimensional analogues of Berger spheres. ==References==
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