Movasati's codimension conjecture for Hodge loci of two intersecting planes

Let k2k\geq 2 be an integer. Let XX be a hypersurface of degree 33 in P2k+1\mathbf{P}^{2k+1} containing two kk-planes Π1\Pi_1 and Π2\Pi_2 whose intersection has dimension k3k-3. For integers a,ba,b with gcd(a,b)=1\gcd(a,b)=1 and ab0ab\neq 0, write NL(a[Π1]+b[Π2])\operatorname{NL}(a[\Pi_1]+b[\Pi_2]) for the Hodge locus where the corresponding linear combination remains of type (k,k)(k,k), and NL([Π1],[Π2])\operatorname{NL}([\Pi_1],[\Pi_2]) for the locus where both classes remain of type (k,k)(k,k). Movasati's conjecture.

codimNL(a[Π1]+b[Π2])=codimNL([Π1],[Π2])1.\operatorname{codim} \operatorname{NL}(a[\Pi_1]+b[\Pi_2])=\operatorname{codim} \operatorname{NL}([\Pi_1],[\Pi_2])-1.

This conjecture concerns the irreducible components and codimensions of Hodge loci associated with linear combinations of the classes of two intersecting planes. The paper presents new evidence for it in the cases where the authors' counterexamples cannot occur, while the general claim is not resolved by the supplied context.

Sources & referencesView supporting material

Primary source

Remke Kloosterman, “On a conjecture on Hodge loci of linear combinations of linear subvarieties”, arXiv:2312.12363 (2025).

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