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Dickinson Peter J.C.(P.J.C.Dickinsonrug.nl) Abstract: We investigate the relation between the cone of $n\times n$ copositive matrices and the approximating cone $\mathcal{K}^1$ introduced by Parrilo. While these cones are known to be equal for $n\leq 4$, we show that for $n \geq 5$ they are unequal. This result is based on the fact that $\mathcal{K}^1$ is not invariant under diagonal scaling. We show that for any copositive matrix which is not the sum of a nonnegative and a positive semidefinite matrix we can find a scaling which is not in $\mathcal{K}^1$. In fact, we show that if all scaled versions of a matrix are contained in $\mathcal{K}^r$ for some fixed $r$, then the matrix must be in $\mathcal{K}^0$. For the $5\times 5$ case, we show the more surprising result that we can scale any copositive matrix $X$ into $\mathcal{K}_5^1$ and in fact that any scaling $D$ such that $(DXD)_{ii}\in\{0,1\}$ for all $i$ yields $DXD \in \mathcal{K}_5^1$. From this we are able to use the cone $\mathcal{K}_5^1$ to check if any order 5 matrix is copositive. Another consequence of this is a complete characterisation of the $5\times 5$ copositive cone in terms of $\mathcal{K}_5^1$. We end the paper by formulating several conjectures. Keywords: copositive cone, Parrilo's approximations, sumofsquares conditions, $5\times 5$ copositive matrices Category 1: Linear, Cone and Semidefinite Programming (Other ) Citation: Preprint, 2012 Download: [PDF] Entry Submitted: 03/19/2012 Modify/Update this entry  
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