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

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,,Hn1H_1,\ldots,H_{n-1} on XX, set

α=H1Hn1.\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 KFKXK_{\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

KFKX.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.

Sources & referencesView supporting material

Primary source

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

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