Rarity of fixed inertia-type prime counts in number fields

Let 1GSn1\leq G\leq S_n be a transitive permutation group, let kk be a number field, and let S\mathcal{S} be the set of number fields (K/k,ψ)(K/k,\psi) whose Galois closures (K^/k,ψ)(\hat{K}/k,\psi) are GG-extensions and for which K=K^G1K=\hat{K}^{G_1}, where G1G_1 is the stabilizer of 11. For a nonempty subset ΩG\Omega\subseteq G closed under invertible powering, let 1(Ω,r)(K)\mathbf{1}_{(\Omega,r)}(K) indicate that exactly rr primes pGp\nmid |G| have inertia subgroup I(p)I(p) intersecting Ω\Omega, and let P(K)P(K) denote the ordering parameter used in the sums. Suppose that idΩ\operatorname{id}\notin\Omega.

Rarity conjecture. For every r=0,1,2,r=0,1,2,\dots, there exists an integer rr' such that

KS(K)<x1(Ω,r)(K)=o(KS(K)<x1(Ω,r)(K)).\sum_{\substack{K\in\mathcal{S}\P(K)<x}}\mathbf{1}_{(\Omega,r)}(K)=o\left(\sum_{\substack{K\in\mathcal{S}\P(K)<x}}\mathbf{1}_{(\Omega,r')}(K)\right).

Moreover, for every r=0,1,2,r=0,1,2,\dots,

KS(K)<x1(Ω,r)(K)=o(KS(K)<x1).\sum_{\substack{K\in\mathcal{S}\P(K)<x}}\mathbf{1}_{(\Omega,r)}(K)=o\left(\sum_{\substack{K\in\mathcal{S}\P(K)<x}}1\right).

These assertions formalize the Malle–Bhargava heuristic that, among fields ordered by P(K)P(K), fields with any fixed number of ramified primes whose inertia meets Ω\Omega are negligible compared with suitable families, and negligible compared with all fields. The source presents both assertions as conjectural; no resolution is supplied.

Sources & referencesView supporting material

Primary source

Weitong Wang, “Distribution of the bad part of class groups”, arXiv:2211.10111 (2025).

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