Singular apparent-space conjecture for half-period configurations

Let τH\tau\in\mathbb{H}, let Σ=Eτ[2]\Sigma=E_\tau[2], and let (α1,,α4)(2Z)4(\alpha_1,\ldots,\alpha_4)\in(2\mathbb{Z})^4. The apparent space APΣ(τ)AP_\Sigma(\tau) parametrizes the relevant apparent data, and SMΣ(τ)\mathcal{SM}_\Sigma(\tau) denotes the corresponding solution space. A point (T,B)(\mathbb{T},B) is called singular when it is a singular point of APΣ(τ)AP_\Sigma(\tau). Singular apparent-space conjecture. If APΣ(τ)AP_\Sigma(\tau) includes a singular point (T,B)(\mathbb{T},B), then SMΣ(τ)\mathcal{SM}_\Sigma(\tau) contains a subset isomorphic to

SL(2,C)/SU(2),SL(2,\mathbb{C})/SU(2),

and the number of such copies is exactly the number of singular points of APΣ(τ)AP_\Sigma(\tau). This predicts that each singular apparent datum contributes one such family of solutions; the paper notes that in the half-period case the singular fiber is of this form, while the general assertion is conjectural.

Sources & referencesView supporting material

Primary source

Erjuan Fu and Chang-Shou Lin, “Classification of spherical metrics on tori with four singularities, I: half periods”, arXiv:2607.19073 (2026).

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