I was reading a paper in which the authors use the fact that any compact simply-connected homogeneous symplectic manifold has non-zero Euler characteristic. They prove it by quoting a theorem by Kostant which implies that the manifold is symplectomorphic to a coadjoint orbit of a semisimple group, then state that compact coadjoint orbits of semisimple groups have non-zero Euler characteristic.


I am looking for a more direct proof of that fact. Do you know some?



Compact simply-connected homogeneous symplectic manifold
Let your manifold be $X=G/H$. First of all, since it is simply connected, we can write it as $K/U$ where $K$ and $U=K\cap H$ are compact in $G$ (Montgomery’s theorem, [1950](//ams.org/mathscinet-getitem?mr=37311)). Next, since $K/U$ is homogeneous symplectic, one knows that $U$ is the centralizer of a torus $S\subset K$.1) In particular $U$ contains any maximal torus containing $S$, i.e. $U$ is an equal rank subgroup of $K$. And finally, one knows that equal rank subgroups satisfy $χ(K/U)\ne0$: e.g. Samelson ([1958](//ams.org/mathscinet-getitem?mr=103509)), or Mostow ([2005](//ams.org/mathscinet-getitem?mr=2174096)).




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1) That is clear, with $S$ the closure of $\exp(\mathbf Rx)$, if we already know that $X\simeq$ the (co)adjoint orbit of some $x\in\mathfrak k^\*\simeq\mathfrak k$. But it can also be proved *a priori* : Borel–Weil ([1954](http://www.numdam.org/item?id=SB_1951-1954__2__447_0), Thm 1), or in more detail Matsushima ([1957](//ams.org/mathscinet-getitem?mr=87177), Thm 1).


