In mathematics, a nearly Kähler manifold is an almost Hermitian manifold M {\displaystyle M} , with almost complex structure J {\displaystyle J} , such that the (2,1)-tensor ∇ J {\displaystyle \nabla J} is skew-symmetric. So,
( ∇ X J ) X = 0 {\displaystyle (\nabla _{X}J)X=0}
for every vector field X {\displaystyle X} on M {\displaystyle M} . In particular, a Kähler manifold is nearly Kähler. The converse is not true. For example, the nearly Kähler six-sphere S 6 {\displaystyle S^{6}} is an example of a nearly Kähler manifold that is not Kähler. The familiar almost complex structure on the six-sphere is not induced by a complex atlas on S 6 {\displaystyle S^{6}} . Usually, non Kählerian nearly Kähler manifolds are called "strict nearly Kähler manifolds". Nearly Kähler manifolds, also known as almost Tachibana manifolds, were studied by Shun-ichi Tachibana in 1959 and then by Alfred Gray from 1970 on. For example, it was proved that any 6-dimensional strict nearly Kähler manifold is an Einstein manifold and has vanishing first Chern class (in particular, this implies spin). In the 1980s, strict nearly Kähler manifolds obtained a lot of consideration because of their relation to Killing spinors: Thomas Friedrich and Ralf Grunewald showed that a 6-dimensional Riemannian manifold admits a Riemannian Killing spinor if and only if it is nearly Kähler. This was later given a more fundamental explanation by Christian Bär, who pointed out that these are exactly the 6-manifolds for which the corresponding 7-dimensional Riemannian cone has holonomy G2. The only compact simply connected 6-manifolds known to admit strict nearly Kähler metrics are S 6 , C P 3 , P ( T C P 2 ) {\displaystyle S^{6},\mathbb {C} \mathbb {P} ^{3},\mathbb {P} (T\mathbb {CP} _{2})} , and S 3 × S 3 {\displaystyle S^{3}\times S^{3}} . Each of these admits such a unique nearly Kähler metric that is also homogeneous, and these examples are in fact the only compact homogeneous strictly nearly Kähler 6-manifolds. However, Foscolo and Haskins recently showed that S 6 {\displaystyle S^{6}} and S 3 × S 3 {\displaystyle S^{3}\times S^{3}} also admit strict nearly Kähler metrics that are not homogeneous. Bär's observation about the holonomy of Riemannian cones might seem to indicate that the nearly-Kähler condition is most natural and interesting in dimension 6. This actually borne out by a theorem of Nagy, who proved that any strict, complete nearly Kähler manifold is locally a Riemannian product of homogeneous nearly Kähler spaces, twistor spaces over quaternion-Kähler manifolds, and 6-dimensional nearly Kähler manifolds. Nearly Kähler manifolds are also an interesting class of manifolds admitting a metric connection with parallel totally antisymmetric torsion. Nearly Kähler manifolds should not be confused with almost Kähler manifolds. An almost Kähler manifold M {\displaystyle M} is an almost Hermitian manifold with a closed Kähler form:
d ω = 0 {\displaystyle d\omega =0} . The Kähler form or fundamental 2-form ω {\displaystyle \omega } is defined by
ω ( X , Y ) = g ( J X , Y ) , {\displaystyle \omega (X,Y)=g(JX,Y),}
where g {\displaystyle g} is the metric on M {\displaystyle M} . The nearly Kähler condition and the almost Kähler condition are essentially exclusive: an almost Hermitian manifold is both nearly Kähler and almost Kahler if and only if it is Kähler.
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