Two-step weighted-volume degeneration conjecture for Fano fibration germs

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Let (π:X→Y,p)(\pi:X\rightarrow Y,p) be a Fano fibration germ. Let Val⁡π,p∗\operatorname{Val}_{\pi,p}^* be the relevant space of valuations and let W\mathbb W denote the weighted volume. For a valuation v∗v_*, write Gr⁡v∗\operatorname{Gr}_{v_*} for the associated graded algebra, and form the schemes and morphisms specified below.

Two-step degeneration conjecture for weighted volume.

  1. There is a unique minimizer v∗v_* of W\mathbb W on Val⁡π,p∗\operatorname{Val}_{\pi,p}^*, which is quasi-monomial.
  2. The graded ring Gr⁡v∗(OY,p)\operatorname{Gr}_{v_*}(\mathcal O_{Y,p}) is finitely generated and ⨁m≥0Gr⁡v∗(Rm)\bigoplus_{m\geq 0}\operatorname{Gr}_{v_*}(R_m) is a finitely generated algebra over Gr⁡v∗(OY,p)\operatorname{Gr}_{v_*}(\mathcal O_{Y,p}).
  3. Let
W=Spec⁡(Gr⁡v∗(OY,p)),Z=Proj⁡W(⨁m≥0Gr⁡v∗(Rm)).W=\operatorname{Spec}(\operatorname{Gr}_{v_*}(\mathcal O_{Y,p})),\qquad Z=\operatorname{Proj}_{W}\left(\bigoplus_{m\geq 0}\operatorname{Gr}_{v_*}(R_m)\right).

Then v∗v_* induces a vector field ξ\xi on ZZ which descends to WW. The natural map π~:Z→W\widetilde\pi:Z\rightarrow W and ξ\xi define a K-semistable polarized Fano fibration. Moreover, v∗v_* is uniquely characterized by this K-semistability property. 4. There is an equivariant test configuration degenerating (π~:Z→W,ξ)(\widetilde\pi:Z\rightarrow W,\xi) to a unique K-polystable polarized Fano fibration (π^:S→C,ξ)(\widehat\pi:\mathcal S\rightarrow\mathcal C,\xi).

The conjecture proposes a canonical algebraic two-step degeneration of a Fano fibration germ, first through weighted-volume minimization and then through a test configuration to a K-polystable object. Its existence, finite generation, uniqueness, and compatibility assertions are open in the stated generality.

References

Primary source

Song Sun and Junsheng Zhang, “Kähler-Ricci shrinkers and Fano fibrations”, arXiv:2410.09661 (2025).

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