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The generalized Miller–Morita–Mumford classes (MMM classes) of a smooth oriented
manifold bundle are defined as the image of the characteristic classes of the
vertical tangent bundle under the Gysin homomorphism. We show that if the
dimension of the manifold is even, then all MMM–classes in rational cohomology
are nonzero for some bundle. In odd dimensions, this is also true with one
exception: the MMM–class associated with the Hirzebruch L–class is always zero.
Moreover, we show that polynomials in the MMM–classes are also nonzero. We
also show a similar result for holomorphic fibre bundles and for unoriented
bundles.
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