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In differential geometry, a hypercomplex manifold is a manifold with the tangent bundle equipped with an action by the algebra of quaternions in such a way that the quaternions define integrable almost complex structures. == Examples == Every hyperkähler manifold is also hypercomplex. The converse is not true. The Hopf surface : (with acting as a multiplication by a quaternion , ) is hypercomplex, but not Kähler, hence not hyperkähler either. To see that the Hopf surface is not Kähler, notice that it is diffeomorphic to a product hence its odd cohomology group is odd-dimensional. By Hodge decomposition, odd cohomology of a compact Kähler manifold are always even-dimensional. In fact H. Wakakuwa proved 〔 . 〕 that on a compact hyperkähler manifold . M. Verbitsky has shown that any compact hypercomplex manifold admitting a Kähler structure is also hyperkähler. In 1988, left-invariant hypercomplex structures on some compact Lie groups were constructed by the physicists Ph. Spindel, A. Sevrin, W. Troost, A. Van Proeyen. In 1992, D. Joyce rediscovered this construction, and gave a complete classification of left-invariant hypercomplex structures on compact Lie groups. Here is the complete list. : : : where denotes an -dimensional compact torus. It is remarkable that any compact Lie group becomes hypercomplex after it is multiplied by a sufficiently big torus. 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「Hypercomplex manifold」の詳細全文を読む スポンサード リンク
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