  We provide a necessary condition for the existence of a compact
Clifford-Klein form of a given homogeneous space of reductive type. The key to
the proof is to combine a result of Kobayashi-Ono with an elementary fact that
certain two different Clifford-Klein forms have the same cohomology ring. We
give some examples, SL(p+q, R)/SO(p, q) (p, q: odd) for instance, of
homogeneous spaces which do not admit compact Clifford-Klein forms.
