Cohen–Lenstra conjecture for class-group pp-parts of number fields

Let KK be a number field of degree nn, let DKD_K be its discriminant, and let ClK,pCl_{K,p} be the pp-part of its class group. Let SS be the set of finite abelian pp-groups, and for A\reSA\re S define

SX7(A)={K0<7DK<X, ClK,pA}K,0<7DK<X1.S_X^{7}(A)=\frac{\left|\left\{K\mid 0<7 D_K<X,\ Cl_{K,p}\cong A\right\}\right|}{\sum_{K,0<7 D_K<X}1}.

For sZ1{}s\in\mathbb{Z}_{\geq 1}\cup\{\infty\}, set

ηs(p)=i=1s(11/pi),\eta_s(p)=\prod_{i=1}^s(1-1/p^i),

and define the CohenLenstra probability measure

μu(A)=η(p)AutAAu.\mu_u(A)=\frac{\eta_\infty(p)}{|\operatorname{Aut}A|\,|A|^u}.

CohenLenstra conjecture. For every ASA\in S,

μ0(A)=limXSX(A),\mu_0(A)=\lim_{X\longrightarrow\infty}S_X^-(A), μ1(A)=limXSX+(A).\mu_1(A)=\lim_{X\longrightarrow\infty}S_X^+(A).

The conjecture predicts limiting distributions for the pp-parts of class groups, with separate measures for negative and positive discriminants. The source gives no resolution status for this general formulation; it notes that the original work treated quadratic fields with p2p\neq 2 separately by signature.

Sources & referencesView supporting material

Primary source

Jack Klys, “The Distribution of p-Torsion in Degree p Cyclic Fields”, arXiv:1610.00226 (2016).

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