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Hypercomplex manifold : ウィキペディア英語版
Hypercomplex manifold
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 I, J, K
define integrable almost complex structures.
== Examples ==

Every hyperkähler manifold is also hypercomplex.
The converse is not true. The Hopf surface
:\bigg(\backslash 0\bigg)/
(with acting
as a multiplication by a quaternion q, |q|>1) 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
S^1\times S^3, 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 \ b_\equiv 0 \ mod \ 4.
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.
:
T^4, SU(2l+1), T^1 \times SU(2l), T^l \times SO(2l+1),

:T^\times SO(4l), T^l \times Sp(l), T^2 \times E_6,
:
T^7\times E^7, T^8\times E^8, T^4\times F_4, T^2\times G_2

where T^i denotes an i-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)
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