Family band width conjecture

Let NBN\to B be a fiber bundle with base BB a connected closed manifold, and with all fibers diffeomorphic to a fixed connected closed manifold ZZ. Suppose there is no fiberwise positive-scalar-curvature metric on NN. Write n1:dimZ2n-1\coloneq\dim Z\geqslant2, with n14n-1\neq4. If the fiber bundle N×[1,1]BN\times[-1,1]\to B admits a metric gg whose fiberwise scalar curvature is bounded below by σ>0\sigma>0, independently of bBb\in B, then the family band width conjecture.

distg{N×{1},N×{1}}2πn1nσ.\mathrm{dist}_g\{N\times\{-1\},N\times\{1\}\}\leqslant2\pi\sqrt{\frac{n-1}{n\sigma}}.

This is the family version of Gromov's band width conjecture, extending the problem from one manifold to a fiber bundle with no fiberwise positive-scalar-curvature metric. The paper proves the estimate for bundles with infinite family A^\widehat{A}-area; the stated general claim remains open.

Sources & referencesView supporting material

Primary source

Chenkai Song, “A Product Formula for Family Indices and Family Band Width Estimates”, arXiv:2507.21594 (2026).

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