The Milnor–Wood inequality for Euler numbers of hyperbolic 4-manifold bundles

Let M=M(e,g)M=M(e,g) be homeomorphic to an R2\mathbb R^2-bundle over a closed orientable surface of genus g>1g>1, with Euler number e=[F],[F]e=\langle[F],[F]\rangle given by the self-intersection of the zero section FF. Milnor–Wood inequality. The inequality

0e2g20\leq |e|\leq 2g-2

is necessary for MM to admit a complete hyperbolic structure. This gives a necessary topological restriction on the Euler number of such bundles; the supplied text does not establish whether the bound is sufficient.

Sources & referencesView supporting material

Primary source

Michael Kapovich, “Intersection pairing on hyperbolic 4-manifolds”, arXiv:2510.16848 (2025).

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