The Gram locus conjecture for real binary tensors

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Let n≥4n\geq 4, and let G(B)\mathcal{G}(\mathcal{B}) denote the Gram locus of real binary tensors, with coordinates d1,…,dnd_1,\ldots,d_n. Define

Q1=∏i=1n(∑j≠idj−di)Q_1=\prod_{i=1}^n\left(\sum_{j\ne i}d_j-d_i\right)

and let Q2Q_2 be the polynomial obtained from

Q2=12∏{i,j,…,k}∈{±1}(id1+jd2+⋯+kdn).Q_2=\frac{1}{2}\prod_{\{i,j,\ldots,k\}\in\{\pm1\}}\left(i\sqrt{d_1}+j\sqrt{d_2}+\cdots+k\sqrt{d_n}\right).

Gram locus conjecture. The Gram locus G(B)\mathcal{G}(\mathcal{B}) is given by

Q1≥Q2and0≤di≤14for i=1,…,n.Q_1\geq Q_2\qquad\text{and}\qquad 0\leq d_i\leq\frac{1}{4}\quad\text{for }i=1,\ldots,n.

This proposes a complete semialgebraic description of the Gram locus for n≥4n\geq4, extending the preceding exact description in the case n=3n=3. The parser supplies no evidence that the conjecture has been resolved.

References

Primary source

Anna Seigal, “Gram Determinants of Real Binary Tensors”, arXiv:1612.04420 (2016).

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