The gap and non-gap conjecture for minimal Terracini loci in projective space

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For n≥3n\ge 3, let T(n,d;x)′\mathbb{T}(n,d;x)' denote the set of minimally Terracini sets of xx points in Pn\mathbb{P}^n with respect to OPn(d)\mathcal{O}_{\mathbb{P}^n}(d): these are sets SS such that h0(I2S(d))>0h^0(\mathcal{I}_{2S}(d))>0, h1(I2S(d))>0h^1(\mathcal{I}_{2S}(d))>0, ⟨S⟩=Pn\langle S\rangle=\mathbb{P}^n, and h1(I2A(d))=0h^1(\mathcal{I}_{2A}(d))=0 for every proper subset A⊊SA\subsetneq S. The gap and non-gap conjecture. Fix an integer c≥2c\ge 2. Then there exists an integer d0(c,n)d_0(c,n) such that for all d≥d0(c,n)d\ge d_0(c,n) there are integers xix_i, 1≤i≤c1\le i\le c, and yiy_i, 1≤i≤c−11\le i\le c-1, such that

x1<y1<x2<y2<⋯<xc−1<yc−1<xc,x_1<y_1<x_2<y_2<\cdots <x_{c-1}<y_{c-1}<x_c,

T(n,d;xi)′≠∅\mathbb{T}(n,d;x_i)'\ne\emptyset for all i=1,…,ci=1,\dots,c, and T(n,d;yi)′=∅\mathbb{T}(n,d;y_i)'=\emptyset for all i=1,…,c−1i=1,\dots,c-1. This predicts arbitrarily many alternating nonempty and empty cardinalities of minimal Terracini loci in higher-dimensional projective spaces, extending the phenomenon proved in the plane.

References

Primary source

Edoardo Ballico and Maria Chiara Brambilla, “Minimal Terracini loci in the plane: gaps and non-gaps”, arXiv:2401.17930 (2024).

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