Difference of the modular function ${\omega }_2(\tau )$, revisited

Abstract

Adapting the analytic method of Gross and Zagier, Roskam proved a prime-factorization formula for the norm of the difference of two level-two Weber singular moduli. Independently, Yang and Yin obtained an equivalent formula using Borcherds lifts. More precisely, the formula concerns the norm of \[ ω_{2}\left(\frac{-1+\sqrt{d_{1}}}{2}\right) - ω_{2}\left(\frac{-1+\sqrt{d_{2}}}{2}\right) \] for coprime negative fundamental quadratic discriminants $d_1,d_2\equiv 1(\mathrm{mod}8)$, where \[ ω_{2}(τ) = 2^{12}\frac{η(2τ)^{24}}{η(τ)^{24}}, \] and $\eta (\tau )$ denotes the Dedekind eta function. In this work, we revisit this formula from the perspective of arithmetic intersection theory and give a new proof.

18 pages