Kollár's fiberwise bimeromorphic model conjecture for canonical families

Let g:XΔg:\mathcal X\to\Delta be a flat, proper, Moishezon morphism over the unit disc Δa\Delta a in C\mathbb C. Assume that X0X_0 has only canonical singularities. A morphism is fiberwise bimeromorphic to another morphism when the corresponding fibers are bimeromorphic. Kollár's fiberwise bimeromorphic model conjecture. The morphism gg is fiberwise bimeromorphic to a flat, projective morphism

gp:XpΔg^{\mathrm p}:\mathcal X^{\mathrm p}\to\Delta

possibly over a smaller disc, such that X0p:=(gp)1(0)X_0^{\mathrm p}:=(g^{\mathrm p})^{-1}(0) has only canonical singularities, Xsp:=(gp)1(s)X_s^{\mathrm p}:=(g^{\mathrm p})^{-1}(s) has terminal singularities for s0s\ne0, and KXPK_{\mathcal X^{\mathrm P}} is Q\mathbb Q-Cartier. Kollár proposed this conjecture as an existence statement for a fiberwise projective model; the supplied text says that the log-terminal case has a positive answer under an additional non-uniruledness assumption, while the canonical case remains under discussion.

Sources & referencesView supporting material

Primary source

Jian Chen, Sheng Rao and I-Hsun Tsai, “Characterization of fiberwise bimeromorphism and specialization of bimeromorphic types I: locally Moishezon case”, arXiv:2506.12670 (2026).

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