Yui–Zagier's factorization conjecture for Weber CM values

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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 d1≡d2≡1(mod8)d_1\equiv d_2\equiv1\pmod 8, define

ω2(τ)=212q∏n>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 m2≡D(mod16)m^2\equiv D\pmod {16}, let pt\mathfrak p_t be the unique prime ideal of FF above 22 such that ord⁡pt(tOF)≥1\operatorname{ord}_{\mathfrak p_t}(t\mathcal O_F)\geq1. Yui–Zagier's factorization conjecture. One has

∑[ai]∈Cl⁡(Ei)log⁡∣ω2(τa1)−ω2(τa2)∣2=∑t=(m+D)/2∣m∣<D, m oddm2≡D(mod16) ∑p inert in E/F1+ord⁡p(tOF)2 ρ(tp−1pt−2)log⁡N⁡(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.

References

Primary source

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

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