Strong uniform boundedness conjecture for Brauer groups of K3 surfaces

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Let kk) be a number field with [k:Q]≤d[k:\mathbb{Q}]\leq d, let L↪ΛL\hookrightarrow\Lambda be a primitive sublattice, and let XX be a K3 surface over kk such that Pic⁡(X‾)≃L\operatorname{Pic}(\overline{X})\simeq L. The quotient Br⁡(X)/Br⁡0(X)\operatorname{Br}(X)/\operatorname{Br}_0(X) is finite, where Br⁡0(X)\operatorname{Br}_0(X) is the subgroup of constant algebras.

Strong uniform boundedness conjecture. There is a constant B=B(d,L)B=B(d,L), independent of XX, such that

#Br⁡(X)Br⁡0(X)<B.\#\frac{\operatorname{Br}(X)}{\operatorname{Br}_0(X)}<B.

This conjecture predicts uniform boundedness in fixed-degree number fields and fixed geometric Picard lattice. The algebraic quotient Br⁡1(X)/Br⁡0(X)\operatorname{Br}_1(X)/\operatorname{Br}_0(X) is already known to be uniformly bounded, so the substantive issue is the transcendental Brauer group. The source gives no resolution status.

References

Primary source

Danny Bragg, Emma Brakkee and Anthony Várilly-Alvarado, “Moduli of lattice-polarized K3 surfaces and boundedness of Brauer groups”, arXiv:2510.11477 (2025).

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