Tosatti's diameter-collapse conjecture for irrational nef isotropic K3 classes

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Let XX be a K3K3 surface and let [α]∈∂C[\alpha]\in\partial\mathcal{C} satisfy

∫Xα2=0\int_X\alpha^2=0

and

R[α]∩H2(X,Q)={0}.\mathbb{R}[\alpha]\cap H^2(X,\mathbb{Q})=\{0\}.

For a class [ω]∈C[\omega]\in\mathcal{C}, let CY([ω])\mathrm{CY}([\omega]) denote the associated Ricci-flat metric.

Tosatti's diameter-collapse conjecture. For every sequence [αi]∈C[\alpha_i]\in\mathcal{C} with [αi]→[α][\alpha_i]\to[\alpha],

diam(X,CY([αi]))→0.\mathrm{diam}(X,\mathrm{CY}([\alpha_i]))\to0.

If true, the Gromov–Hausdorff limit of the corresponding metric spaces would be a point. The source gives no resolution of the conjecture.

References

Primary source

Valentino Tosatti, “Ricci-flat metrics on Calabi-Yau manifolds”, arXiv:2509.25607 (2025).

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