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v2007.09.13 - Convex Optimization

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638 APPENDIX E. PROJECTION10 0dist(P 2 X i , S n +)10 −10 2 4 6 8 10 12 14 16 18iFigure 126: Distance (confer (1840)) between PSD cone and iterate (1843) inaffine subset A (1836) for Laurent’s completion problem; initially, decreasinggeometrically.Laurent gives a problem for which no positive semidefinite completionexists: [171]⎡ ⎤1 1 ? 0⎢ 1 1 1 ?⎥⎣ ? 1 1 1 ⎦ (1845)0 ? 1 1Initializing unknowns to 0, by alternating projection we find the constrainedmatrix closest to the positive semidefinite cone,⎡⎤1 1 0.5454 0⎢ 1 1 1 0.5454⎥⎣ 0.5454 1 1 1 ⎦ (1846)0 0.5454 1 1and we find the positive semidefinite matrix closest to the affine subset A(1836): ⎡⎤1.0521 0.9409 0.5454 0.0292⎢ 0.9409 1.0980 0.9451 0.5454⎥⎣ 0.5454 0.9451 1.0980 0.9409 ⎦ (1847)0.0292 0.5454 0.9409 1.0521These matrices (1846) and (1847) attain the Euclidean distance dist(A , S+) n .Convergence is illustrated in Figure 126.

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