Difference of the modular function $ω_{2}(τ)$, revisited

Wei-Lun Tsai; Dongxi Ye

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arXiv · 2608.29788

Difference of the modular function $ω_{2}(τ)$, revisited

Wei-Lun Tsai·Dongxi Ye

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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 $d_{1},d_{2}\equiv1\pmod 8$, where \[ ω_{2}(τ) = 2^{12}\frac{η(2τ)^{24}}{η(τ)^{24}}, \] and $η(τ)$ denotes the Dedekind eta function. In this work, we revisit this formula from the perspective of arithmetic intersection theory and give a new proof.

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Wei-Lun Tsai, Dongxi Ye. 2026-08-30. Difference of the modular function $ω_{2}(τ)$, revisited. https://arxiv.org/abs/2608.29788

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