Orthogonal splitting type dimension conjecture for general covers

From papers

Let r1r \geq 1 and k3k \geq 3. For a general chain

C~fChP1,\widetilde{C} \xrightarrow{f} C \xrightarrow{h} \mathbb{P}^1,

where ff is an étale double cover, CC has genus gg, and hh has degree kk, define the orthogonal splitting type locus

Ua(C,f,h)={LP(C,f)hfLOP1(1)OP1(a)OP1(a)}.U^{\vec{a}}(C,f,h)=\{\mathcal{L}\in P(C,f)\mid h_*f_*\mathcal{L}\otimes\mathcal{O}_{\mathbb{P}^1}(1)\cong \mathcal{O}_{\mathbb{P}^1}(\vec{a})\oplus\mathcal{O}_{\mathbb{P}^1}(-\vec{a})\}.

Orthogonal splitting type dimension conjecture. Its dimension is

dimUa(C,f,h)=g1h1(Endq(OP1(a)OP1(a)))=g1i>j(max{0,ai+aj1}+max{0,aiaj1}).\begin{aligned} \mathrm{dim}\,U^{\vec{a}}(C,f,h)&=g-1-h^1\bigl(\operatorname{End}^q(\mathcal{O}_{\mathbb{P}^1}(\vec{a})\oplus\mathcal{O}_{\mathbb{P}^1}(-\vec{a}))\bigr)\\ &=g-1-\sum_{i>j}\left(\max\{0,a_i+a_j-1\}+\max\{0,a_i-a_j-1\}\right). \end{aligned}

Here Endq\operatorname{End}^q is the sheaf of endomorphisms preserving the relevant orthogonal structure. This more general conjecture is stated as implying the conjectural dimension formula for Vr(C,f)V^r(C,f) in the kk-gonal setting. The source does not provide a resolution of this conjecture.

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Sources & referencesView supporting material

Primary source

David Jensen, “Prym-Brill-Noether Theory for General Covers”, arXiv:2607.01173 (2026).

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