The unlikely intersection bound with multiplicities

Let KK be a number field, let η\eta be the generic point of SpecOK\operatorname{Spec}\mathcal{O}_K, and let VGm,OKn\mathcal{V}\subseteq\mathbb{G}^n_{m,\mathcal{O}_K} be an integral closed subscheme dominating SpecOK\operatorname{Spec}\mathcal{O}_K. Put V=VηV=\mathcal{V}_\eta. Suppose that VV is not contained in any proper algebraic subgroup of Gm,Kn\mathbb{G}^n_{m,K}. For a flat subgroup scheme H\mathcal{H}, write C(H)\mathcal{C}(\mathcal{H}) for its complexity, and let mult(Z)\operatorname{mult}(\mathcal{Z}) denote the multiplicity of an irreducible component Z\mathcal{Z} of the intersection.

Unlikely intersection bound. For every ε>0\varepsilon>0, there exists c=c(n,V,ε,K)Rc=c(n,\mathcal{V},\varepsilon,K)\in\mathbb{R} such that, for every flat subgroup scheme HGm,OKn\mathcal{H}\subseteq\mathbb{G}^n_{m,\mathcal{O}_K} satisfying dimH+dimV<n+1\dim\mathcal{H}+\dim\mathcal{V}<n+1,

Zmult(Z)deg(Zred)logN(Z)εC(H)dimV+c.\sum_{\mathcal{Z}}\operatorname{mult}(\mathcal{Z})\deg(\mathcal{Z}_{\operatorname{red}})\log N(\mathcal{Z})\leq\varepsilon\mathcal{C}(\mathcal{H})^{\dim\mathcal{V}}+c.

The sum is over all irreducible components of VH\mathcal{V}\cap\mathcal{H} that do not dominate SpecOK\operatorname{Spec}\mathcal{O}_K. This is the unlikely-intersection refinement of the general bound: the generic-fiber noncontainment hypothesis and strict dimension inequality permit an arbitrarily small coefficient of the complexity term. The source presents it as conjectural and discusses particular instances and related known results.

Sources & referencesView supporting material

Primary source

Francesco Campagna, Gabriel Andreas Dill and Robert Wilms, “Arithmetic unlikely intersections in powers of the multiplicative group”, arXiv:2607.15741 (2026).

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