Modular congruence criterion for non-unirational surfaces

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Let YY be a surface satisfying the projective-resolution and congruence assumptions, and let Y~\widetilde{Y} be a fixed minimal smooth model. Suppose that for some NNN\in\mathbb{N}, a subgroup ΓPSL2(Z)\Gamma\leq\operatorname{PSL}_2(\mathbb{Z}), and a positive integer BB with #Σ0(B)10\#\Sigma_0(B)\geq 10, there is a newform f=k=1bkqkS3new(Γ(N))f=\sum_{k=1}^{\infty}b_kq^k\in S_3^{\mathrm{new}}(\Gamma(N)) satisfying bp#Yp(Fp)1(modp)b_p\equiv\#Y_p(\mathbb{F}_p)-1\pmod p for every pΣ(B)p\in\Sigma(B). Modular congruence criterion. If this condition holds, then YY is birationally equivalent to a smooth surface Y~\widetilde{Y} with h2,0(Y~)1h^{2,0}(\widetilde{Y})\geq 1. In particular, YY is not unirational. The criterion is intended as a point-counting consistency check for identifying surfaces whose smooth models have nontrivial holomorphic 22-forms; the supplied text does not establish its status beyond this asserted implication.

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Primary source

Dino Festi and Bert van Geemen, “A Calabi-Yau threefold coming from two black holes”, arXiv:2207.01936 (2022).

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