Higher-factor defective-span conjecture beyond boundary format

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 TVT\in V a general tensor. Higher-factor defective-span conjecture. If k3k\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,

ZTP(HT)\langle Z_T\rangle\subsetneq\mathbb{P}(H_T)

has codimension j=1k(n1nj)(n+2)j=1k(n2nj)\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.

Sources & referencesView supporting material

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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