Orthogonal Grassmannian decomposition conjecture for a pencil of quadrics

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Let ⟨Q1,Q2⟩\langle Q_1,Q_2\rangle be a pencil of quadrics in Pn+2\mathbb{P}^{n+2} whose base locus is smooth in an odd-dimensional projective space. Let CC be the hyperelliptic curve associated with the pencil, and let OGr⁡(2,Q)\operatorname{OGr}(2,\mathfrak{Q}) denote the relative orthogonal Grassmannian of isotropic 2-planes. Pencil conjecture. There is a semiorthogonal decomposition

Db(OGr⁡(2,Q))=⟨Db(C),…,Db(C)⏟n+1 times,Db(C),…,Db(C)⏟(n−1)(n+1) times⟩.\mathrm{D}^{\mathrm{b}}(\operatorname{OGr}(2,\mathfrak{Q}))=\left\langle\underbrace{\mathrm{D}^{\mathrm{b}}(C),\dots,\mathrm{D}^{\mathrm{b}}(C)}_{n+1\text{ times}},\underbrace{\mathrm{D}^{\mathrm{b}}(\mathbb{C}),\dots,\mathrm{D}^{\mathrm{b}}(\mathbb{C})}_{(n-1)(n+1)\text{ times}}\right\rangle.

This conjecture is obtained formally by subtracting the conjectural decomposition for F1(X)F_1(X) from the known decomposition for the Hilbert square of the base locus. The paper interprets it as a relativization of full exceptional collections for Grassmannians of isotropic lines; no resolution status is supplied.

References

Primary source

Saket Shah, “Flips for spaces of quadrics on del Pezzo varieties”, arXiv:2602.07366 (2026).

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