Phantom-category existence and deformation conjecture for critical blowups of Hirzebruch surfaces

From papers

Let nn be a nonnegative integer, set d(n)=6+max3,nd(n)=6+\max\\{3,n\\}, and let XX be obtained by blowing up Fn\mathbf{F}_n at d(n)d(n) general points. Let Cn\mathcal{C}_n be a subcategory of Db(X)\operatorname{{D^b}}(X), and let H1(X,TX)\mathrm{H}^1(X,\mathcal{T}_X) denote the first cohomology of the tangent sheaf while HH2(Cn)\operatorname{HH}^2(\mathcal{C}_n) denotes the second Hochschild cohomology of Cn\mathcal{C}_n. Phantom-category existence and deformation conjecture. The category Db(X)\operatorname{{D^b}}(X) has a phantom subcategory Cn\mathcal{C}_n orthogonal to an exceptional collection of line bundles of maximal length. Furthermore, the map

H1(X,TX)HH2(Cn)\mathrm{H}^1(X,\mathcal{T}_X) \to \operatorname{HH}^2(\mathcal{C}_n)

is an isomorphism, so that Cn≄Cm\mathcal{C}_n\not\simeq\mathcal{C}_m for nmn\ne m. This conjecture predicts distinct phantom categories at the critical number of blowups, distinguished by their Hochschild cohomology and deformation behavior; no resolution is given in the source.

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Primary source

Kimoi Kemboi, Daniel Krashen, Tianle Liu, Yeqin Liu, Eoin Mackall, Svetlana Makarova, Alexander Perry, Antonios-Alexandros Robotis and Sridhar Venkatesh, “A Looming of phantoms”, arXiv:2511.05381 (2025).

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