Cohomological lower-bound conjecture for non-Fano hypersurfaces

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Let kk be an algebraically closed field and let i ⁣:X=V(f)↪Pkni\colon X=V(f)\hookrightarrow\mathbb{P}^n_k be a closed embedding of a smooth irreducible hypersurface of degree dd. For a bounded complex C\mathcal C of coherent sheaves on XX, define

ρ(C)=∑r=0nh(Pkn,i∗(C)⊗ΩPknr(r)),\rho(\mathcal C)=\sum_{r=0}^n h(\mathbb{P}^n_k,i_*(\mathcal C)\otimes\Omega_{\mathbb{P}^n_k}^r(r)),

where h(Y,C)=∑j∈Zdim⁡kRjΓ(Y,C)h(Y,\mathcal C)=\sum_{j\in\mathbb{Z}}\dim_k\mathbf{R}^j\Gamma(Y,\mathcal C). Put a≔n+1−da\coloneqq n+1-d. Cohomological lower-bound conjecture. If a≤0a\leq0, equivalently if XX is not Fano, then for any nonzero object C∈D⁡b⁡(X)\mathcal C\in\operatorname{D}^{\operatorname{b}}(X) one has

ρ(C)≥2e+1,e≔⌊n2⌋.\rho(\mathcal C)\geq 2^{e+1},\qquad e\coloneqq\left\lfloor\frac{n}{2}\right\rfloor.

The conjecture proposes a uniform lower bound on the cohomological size of every nonzero derived-category object on a non-Fano smooth hypersurface; the source states that the assertion is false without the assumption a≤0a\leq0.

References

Primary source

Michael K. Brown and Mark E. Walker, “Ranks of matrix factorizations and sheaf cohomology”, arXiv:2412.01060 (2025).

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