The rational orthogonal realization conjecture for rational zero-patterns

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Let XX be an m×nm\times n matrix over a field. Its zero-pattern (or support) is the (0,1)(0,1)-matrix X‾\underline{X} defined by

X‾i,j={1,Xi,j≠0,0,Xi,j=0.\underline{X}_{i,j}=\begin{cases}1,&X_{i,j}\neq0,\\0,&X_{i,j}=0.\end{cases}

Let O(n)O(n) denote the real orthogonal n×nn\times n matrices, and let On(Q)O_n(\mathbb{Q}) denote the orthogonal n×nn\times n matrices with rational entries. The rational orthogonal realization conjecture. For any Y∈O(n)Y\in O(n) there exists Z∈On(Q)Z\in O_n(\mathbb{Q}) such that Y‾=Z‾\underline{Y}=\underline{Z}.

This is the rational analogue of the preceding unitary-versus-real zero-pattern conjecture: it asks whether every zero-pattern realized by a real orthogonal matrix is also realized by a rational orthogonal matrix. The abstract records verification of this claim for matrix size n≤5n\leq5, while no general resolution is supplied.

References

Primary source

Dragomir Z. Djokovic, Simone Severini and Ferenc Szollosi, “Rational Orthogonal versus Real Orthogonal”, arXiv:0903.2853 (2009).

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