Existence of DODDs for arbitrary matrices in sufficiently large dimensions

Let XRm×n\mathbf{X}\in\mathbb{R}^{m\times n}, and for an integer dm,nd\geq m,n let its 0-inflation be the d×dd\times d matrix obtained by placing X\mathbf{X} in the top-left corner and filling the remaining entries with zeros. A DODD is a decomposition involving diagonal matrices and an orthogonal matrix; write the corresponding rectangular block as Q\overline{\mathbf{Q}}. Equivalently, let CRm×d\mathbf{C}\in\mathbb{R}^{m\times d} have orthonormal rows and let FRd×n\mathbf{F}\in\mathbb{R}^{d\times n} have orthonormal columns.

DODD existence conjecture. The d×dd\times d 0-inflation of any matrix XRm×n\mathbf{X}\in\mathbb{R}^{m\times n} admits a DODD for some sufficiently large dm,nd\geq m,n. Thus every such matrix has a decomposition

X=ΛQM,\mathbf{X}=\boldsymbol\Lambda\overline{\mathbf{Q}}\mathbf{M},

where ΛRm×m\boldsymbol\Lambda\in\mathbb{R}^{m\times m} and MRn×n\mathbf{M}\in\mathbb{R}^{n\times n} are diagonal, and QRm×n\overline{\mathbf{Q}}\in\mathbb{R}^{m\times n} is the top-left block of an orthogonal matrix QRd×d\mathbf{Q}\in\mathbb{R}^{d\times d}. Equivalently,

X=ΛCFM.\mathbf{X}=\boldsymbol\Lambda\mathbf{C}^{\top}\mathbf{F}\mathbf{M}.

The conjecture would establish existence of such orthogonal-diagonal decompositions for every rectangular matrix after sufficiently large zero inflation. The source supports the conjecture only with numerical experiments, so its resolution remains open.

Sources & referencesView supporting material

Primary source

Karim Halaseh, Tommi Muller and Elina Robeva, “Orthogonal Decomposition of Tensor Trains”, arXiv:2010.04202 (2021).

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