Mathematical Programming

, Volume 109, Issue 2–3, pp 283–317 | Cite as

Sums of random symmetric matrices and quadratic optimization under orthogonality constraints

  • Arkadi Nemirovski


Let B i be deterministic real symmetric m × m matrices, and ξ i be independent random scalars with zero mean and “of order of one” (e.g., \(\xi_{i}\sim \mathcal{N}(0,1)\)). We are interested to know under what conditions “typical norm” of the random matrix \(S_N = \sum_{i=1}^N\xi_{i}B_{i}\) is of order of 1. An evident necessary condition is \({\bf E}\{S_{N}^{2}\}\preceq O(1)I\), which, essentially, translates to \(\sum_{i=1}^{N}B_{i}^{2}\preceq I\); a natural conjecture is that the latter condition is sufficient as well. In the paper, we prove a relaxed version of this conjecture, specifically, that under the above condition the typical norm of S N is \(\leq O(1)m^{{1\over 6}}\): \({\rm Prob}\{||S_N||>\Omega m^{1/6}\}\leq O(1)\exp\{-O(1)\Omega^2\}\) for all Ω > 0 We outline some applications of this result, primarily in investigating the quality of semidefinite relaxations of a general quadratic optimization problem with orthogonality constraints \({\rm Opt} = \max\limits_{X_{j}\in{\bf R}^{m\times m}}\left\{F(X_1,\ldots ,X_k): X_jX_j^{\rm T}=I,\,j=1,\ldots ,k\right\}\), where F is quadratic in X =  (X 1,... ,X k ). We show that when F is convex in every one of X j , a natural semidefinite relaxation of the problem is tight within a factor slowly growing with the size m of the matrices \(X_j : {\rm Opt}\leq {\rm Opt}(SDP)\leq O(1) [m^{1/3}+\ln k]{\rm Opt}\).


Large deviations Random perturbations of linear matrix inequalities Semidefinite relaxations Orthogonality constraints Procrustes problem 

Mathematics Subject Classification (2000)

60F10 90C22 90C25 90C59 


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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  1. 1.ISYE, Georgia Institute of TechnologyAtlantaUSA

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