Conjecture on smooth and oversized Hodge loci for cubic hypersurfaces

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Let X0X_0 be a cubic hypersurface in the full family of cubic hypersurfaces, and let Pn2\mathbb{P}^{\frac{n}{2}} and Pˇn2\check{\mathbb{P}}^{\frac{n}{2}} be linear cycles in X0X_0. For a Hodge locus VγV_\gamma, write V[Pn2]−[Pˇn2]V_{[\mathbb{P}^{\frac{n}{2}}]-[\check{\mathbb{P}}^{\frac{n}{2}}]} and Vr[Pn2]−rˇ[Pˇn2]V_{r[\mathbb{P}^{\frac{n}{2}}]-\check r[\check{\mathbb{P}}^{\frac{n}{2}}]} for the loci associated with the indicated cohomology classes. Smooth and oversized Hodge-locus conjecture. For the full family of cubic hypersurfaces, the following hold: (1) if Pn2∩Pˇn2=Pn2−2\mathbb{P}^{\frac{n}{2}}\cap\check{\mathbb{P}}^{\frac{n}{2}}=\mathbb{P}^{\frac{n}{2}-2}, then V[Pn2]−[Pˇn2]V_{[\mathbb{P}^{\frac{n}{2}}]-[\check{\mathbb{P}}^{\frac{n}{2}}]} is smooth and larger than the deformation space of the triple (X0,Pn2,Pˇn2)(X_0,\mathbb{P}^{\frac{n}{2}},\check{\mathbb{P}}^{\frac{n}{2}}); (2) if Pn2∩Pˇn2=Pn2−3\mathbb{P}^{\frac{n}{2}}\cap\check{\mathbb{P}}^{\frac{n}{2}}=\mathbb{P}^{\frac{n}{2}-3} and ∣r∣,∣rˇ∣∈N|r|,|\check r|\in\mathbb{N}, then Vr[Pn2]−rˇ[Pˇn2]V_{r[\mathbb{P}^{\frac{n}{2}}]-\check r[\check{\mathbb{P}}^{\frac{n}{2}}]} is smooth and larger than the deformation space of the same triple, with dimension difference 11. The conjecture concerns excess-dimensional Hodge loci in the full cubic family; the supplied status evidence says that the integral and rational Hodge conjecture are proved in this case, so this candidate is recorded as solved.

References

Primary source

Hossein Movasati, “Hodge cycles for cubic hypersurfaces”, arXiv:1902.00831 (2019).

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