The MRC-foliation characterization for varieties with nef anti-canonical bundle

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Let XX be a smooth nn-dimensional projective variety with nef anti-canonical bundle, and let F\mathcal{F} be a foliation on XX. For some ample divisors H1,…,Hn−1H_1,\ldots,H_{n-1} on XX, set

α=H1⋯Hn−1.\alpha=H_1\cdots H_{n-1}.

Here μαmin⁡(F)\mu_{\alpha}^{\min}(\mathcal{F}) denotes the minimal slope of F\mathcal{F} with respect to α\alpha, and KF≡KXK_{\mathcal{F}}\equiv K_X denotes numerical equivalence of canonical classes. MRC-foliation characterization. The foliation F\mathcal{F} is induced by the maximal rationally connected (MRC) fibration of XX if and only if

μαmin⁡(F)>0\mu_{\alpha}^{\min}(\mathcal{F})>0

and

KF≡KX.K_{\mathcal{F}}\equiv K_X.

This conjecture proposes a numerical characterization of the foliation induced by the MRC fibration when the anti-canonical bundle of XX is nef. The preceding results establish related statements for regular foliations and morphisms under stronger assumptions, while the claimed equivalence is not resolved here.

References

Primary source

Masataka Iwai, “Almost nef regular foliations and Fujita's decomposition of reflexive sheaves”, arXiv:2007.13954 (2021).

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