\(5 \times 5\) completely positive matrices
A matrix is said to be completely positive if \(A=BB^T\) where \(B\) is nonnegative (entrywise); this implies double nonnegativity, that is, both entrywise nonnegativity and positive semidefiniteness. This paper gives a list of sufficient conditions for the converse for \(5\times 5\) matrices, which involve Schur complements. Two conjectures are stated which would complete the characterization of \(5\times 5\) completely positive matrices.
scientific article; zbMATH DE number 2126342