Divisibility conjecture for exponents of modular-unit basis elements

Let NN be a positive integer, let LL be the level-dependent integer used for the Galois action, and let DN\mathcal{D}_N and Fm,hF_{m,h} be the indexing set and modular-unit basis introduced in the paper. Let (m)\ell(m) be the associated integer and let φ\varphi denote Euler's totient function. Suppose n2n\geq 2 and

F=mDNh=0φ((m))1Fm,he(m,h),e(m,h)Z,F=\prod_{m\in\mathcal{D}_N}\prod_{h=0}^{\varphi(\ell(m))-1}F_{m,h}^{e(m,h)},\qquad e(m,h)\in\mathbf{Z},

is a modular unit on X0(N)X_0(N). Assume that the order of FF at every cusp is divisible by nn, and that (σs(F)/F)1/n(\sigma_s(F)/F)^{1/n} is a modular unit on X0(N)X_0(N) for every integer ss coprime to LL. The divisibility conjecture. Then e(m,h)e(m,h) is divisible by nn for every mDNm\in\mathcal{D}_N^* and every h0h\ne0. This is proposed as the key divisibility statement underlying the strategy for Yoo's conjecture; the paper does not provide a general proof.

Sources & referencesView supporting material

Primary source

Hwajong Yoo and Myungjun Yu, “The rational cuspidal subgroup of J_0(N)”, arXiv:2504.12564 (2025).

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