In differential geometry, a hypercomplex manifold is a manifold with the tangent bundle equipped with an action by the algebra of quaternions define integrable almost complex structures.

If the almost complex structures are instead not assumed to be integrable, the manifold is called quaternionic, or almost hypercomplex.

Examples

Every hyperkähler manifold is also hypercomplex. The converse is not true. The Hopf surface

) is hypercomplex, but not Kähler, hence not hyperkähler either. To see that the Hopf surface is not Kähler, notice that it is diffeomorphic hence its odd cohomology group is odd-dimensional. By Hodge decomposition, odd cohomology of a compact Kähler manifold are always even-dimensional. In fact Hidekiyo Wakakuwa proved that on a compact hyperkähler manifold. Misha 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 Philippe Spindel, Alexander Sevrin, Walter Troost, and Antoine Van Proeyen. In 1992, Dominic Joyce rediscovered this construction, and gave a complete classification of left-invariant hypercomplex structures on compact Lie groups. Here is the complete list.

-dimensional compact torus.

It is remarkable that any compact Lie group becomes hypercomplex after it is multiplied by a sufficiently big torus.

Basic properties

, the Hopf surface and the K3 surface.

Much earlier (in 1955) Morio Obata studied affine connection associated with almost hypercomplex structures (under the former terminology of Charles Ehresmann of almost quaternionic structures). His construction leads to what Edmond Bonan called the Obata connection which is torsion free are integrable and in this case the manifold is hypercomplex.

Twistor spaces

. Each of these quaternions gives a complex structure on a hypercomplex manifold M. This complex structure is integrable, as follows from Obata's theorem (this was first explicitly proved by Dmitry Kaledin). This complex manifold is called the twistor space. If M.

See also

• Quaternionic manifold

• Hyperkähler manifold

References

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