Higher-factor defective-span conjecture beyond boundary format

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Let n=∑j=1knjn=\sum_{j=1}^k n_j and consider a (k+1)(k+1)-tensor space VV of format (n1+1,…,nk+1,n+2)(n_1+1,\dots,n_k+1,n+2), with T∈VT\in V a general tensor. Higher-factor defective-span conjecture. If k≥3k\geq3, then

⟨ZT⟩=P(HT)\langle Z_T\rangle=\mathbb{P}(H_T)

except in the cases (2,2,2,5)(2,2,2,5), (2,2,3,6)(2,2,3,6), and (2,2,4,7)(2,2,4,7). In these latter cases,

⟨ZT⟩⊊P(HT)\langle Z_T\rangle\subsetneq\mathbb{P}(H_T)

has codimension ∏j=1k(n−1nj)−(n+2)∏j=1k(n−2nj)\prod_{j=1}^k\binom{n-1}{n_j}-(n+2)\prod_{j=1}^k\binom{n-2}{n_j}. This conjecture is the higher-factor consequence of the maximal-rank conjecture for αT\alpha_T and identifies the listed formats as the only predicted defective cases.

References

Primary source

Ettore Teixeira Turatti and Emanuele Ventura, “On defective spans of singular vector tuples beyond the boundary format”, arXiv:2605.22118 (2026).

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