Milman's measure-preserving contraction conjecture for positively Ricci-curved spheres

Let (Mn,g)(M^n,g) be a smooth Riemannian manifold diffeomorphic to Sn\mathbb S^n and suppose that

Ricg(n1)g.\operatorname{Ric}_g\geq(n-1)g.

Milman's contraction conjecture. There exists a map

T:(Sn,gcan)(M,g)T:(\mathbb S^n,g_{\mathrm{can}})\longrightarrow(M,g)

such that

T#dvolgcanSn=dvolgvolg(M),Lip(T)1.T_\#\frac{\,\mathrm d\operatorname{vol}_{g_{\mathrm{can}}}}{|\mathbb S^n|}=\frac{\,\mathrm d\operatorname{vol}_g}{\operatorname{vol}_g(M)},\qquad \operatorname{Lip}(T)\leq1.

This conjecture is a spherical Riemannian analogue of Caffarelli's contraction theorem. The paper proves it in dimension two using inverse mean curvature flow, while the general-dimensional statement remains open.

Sources & referencesView supporting material

Primary source

Bang-Xian Han and Zhuo-Nan Zhu, “Contraction Maps Generated by Inverse Mean Curvature Flow”, arXiv:2607.27711 (2026).

Additional references

2 papers in this index state this conjecture (2025–2026). The statement above is taken from the most recent of them; the others are arXiv:2512.01496.

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