Yui–Zagier's factorization conjecture for Weber CM values

Let Ei=Q(di)E_i=\mathbb{Q}(\sqrt{d_i}) be imaginary quadratic fields of fundamental discriminants did_i with (d1,d2)=1(d_1,d_2)=1, let F=Q(D)F=\mathbb{Q}(\sqrt D) with D=d1d2D=d_1d_2, and let E=Q(d1,d2)E=\mathbb{Q}(\sqrt{d_1},\sqrt{d_2}). Let τai\tau_{\mathfrak a_i} be the CM points associated to integral ideal representatives ai\mathfrak a_i of ideal classes in Cl(Ei)\operatorname{Cl}(E_i), chosen with norm prime to 22, and let ρ(a)\rho(\mathfrak a) count ideals of EE with relative norm a\mathfrak a. For d1d21(mod8)d_1\equiv d_2\equiv1\pmod 8, define

ω2(τ)=212qn>0(1+qn)24=212Δ(2τ)Δ(τ).\omega_2(\tau)=2^{12}q\prod_{n>0}(1+q^n)^{24}=2^{12}\frac{\Delta(2\tau)}{\Delta(\tau)}.

For t=(m+D)/2t=(m+\sqrt D)/2 satisfying m<D|m|<\sqrt D, mm odd, and m2D(mod16)m^2\equiv D\pmod {16}, let pt\mathfrak p_t be the unique prime ideal of FF above 22 such that ordpt(tOF)1\operatorname{ord}_{\mathfrak p_t}(t\mathcal O_F)\geq1. Yui–Zagier's factorization conjecture. One has

[ai]Cl(Ei)logω2(τa1)ω2(τa2)2=t=(m+D)/2m<D, m oddm2D(mod16) p inert in E/F1+ordp(tOF)2ρ(tp1pt2)logN(p).\sum_{[\mathfrak a_i]\in\operatorname{Cl}(E_i)}\log\left|\omega_2(\tau_{\mathfrak a_1})-\omega_2(\tau_{\mathfrak a_2})\right|^2 =\sum_{\substack{t=(m+\sqrt D)/2\\ |m|<\sqrt D,\ m\text{ odd}\\ m^2\equiv D\pmod {16}}}\ \sum_{\mathfrak p\text{ inert in }E/F}\frac{1+\operatorname{ord}_{\mathfrak p}(t\mathcal O_F)}2\,\rho(t\mathfrak p^{-1}\mathfrak p_t^{-2})\log\operatorname{N}(\mathfrak p).

This is the a=24a=24 case of the conjectural factorization proposed by Yui and Zagier for differences of CM values. The paper discusses a potential obstruction: the big CM cycle used in the Bruinier–Kudla–Yang framework may generally be larger than the ideal class groups appearing in the proposed formula, so the conjecture's validity remains unclear.

Sources & referencesView supporting material

Primary source

Tonghai Yang and Hongbo Yin, “Difference of modular functions and their CM value factorization”, arXiv:1711.02983 (2017).

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