The rationality conjecture for cubic fourfolds with associated K3 surfaces

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Let XX be a smooth cubic fourfold in P5\mathbf{P}^5. Let A(X)=H4(X,Z)∩H2,2(X)A(X)=H^4(X,\mathbf{Z})\cap H^{2,2}(X), and let Cd\mathcal{C}_d denote the Hassett divisor of special cubic fourfolds with a labelled discriminant-dd rank-two lattice. The numerical condition (*) is

d>6andd is not divisible by 4,9, or by a prime p≡2(mod3).d>6\quad\text{and}\quad d\text{ is not divisible by }4,9\text{, or by a prime }p\equiv 2\pmod{3}.

Rationality conjecture. A smooth cubic fourfold XX is rational if and only if X∈CdX\in\mathcal{C}_d for some dd satisfying (*).

This is the rationality criterion suggested by the relationship between special cubic fourfolds and associated K3 surfaces. The source gives no resolution status for this assertion.

References

Primary source

Claudio Pedrini, “K3 surfaces associated to a cubic fourfold”, arXiv:2405.05074 (2024).

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