The Euler-characteristic discrepancy conjecture for degeneracy-locus Hilbert squares

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Let m≥1m\geq1, let s∈{m+3,…,2m+3}s\in\{m+3,\ldots,2m+3\}, and set n=2s−2m−3n=2s-2m-3. Let Mm,s\mathsf{M}_{m,s} be the degeneracy locus considered in the paper and let Zn,s,mZ_{n,s,m} be the associated variety. Euler-characteristic discrepancy conjecture. One has

etop(Hilb⁡2(Mm,s))−etop(Zn,s,m)=(−1)dim⁡(Mm,s)1s(2s−m−2s−1)(2s−m−3s−1).e_{\mathrm{top}}(\operatorname{Hilb}^2(\mathsf{M}_{m,s}))-e_{\mathrm{top}}(Z_{n,s,m})=(-1)^{\dim(\mathsf{M}_{m,s})}\frac{1}{s}\binom{2s-m-2}{s-1}\binom{2s-m-3}{s-1}.

The formula is motivated by explicit examples, where the discrepancy equals the number of contracted special fibres; its validity in the stated parameter range remains open.

References

Primary source

Enrico Fatighenti, Francesco Meazzini, Giovanni Mongardi and Andrea T. Ricolfi, “Hilbert squares of degeneracy loci”, arXiv:2204.00437 (2022).

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