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v2007.09.13 - Convex Optimization

v2007.09.13 - Convex Optimization

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2.9. POSITIVE SEMIDEFINITE (PSD) CONE 99γsvec ∂ S 2 +[ α ββ γ]α√2βMinimal set of generators are the extreme directions: svec{yy T | y ∈ R M }Figure 31: Truncated boundary of PSD cone in S 2 plotted in isometricallyisomorphic R 3 via svec (47); courtesy, Alexandre W. d’Aspremont. (Plottedis 0-contour of smallest eigenvalue (162). Lightest shading is closest.Darkest shading is furthest and inside shell.) Entire boundary can beconstructed from an aggregate of rays (2.7.0.0.1) emanating exclusivelyfrom the origin, { √κ 2 [z12 2z1 z 2 z2 2 ] T | κ∈ R } . In this dimension the coneis circular (2.9.2.5) while each and every ray on boundary corresponds toan extreme direction, but such is not the case in any higher dimension(confer Figure 15). PSD cone geometry is not as simple in higher dimensions[20,II.12], although for real matrices it is self-dual (321) in ambient spaceof symmetric matrices. [144,II] PSD cone has no two-dimensional faces inany dimension, and its only extreme point resides at the origin.

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