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v2007.09.13 - Convex Optimization

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160 CHAPTER 2. CONVEX GEOMETRYΓ 4Γ 2Γ 3K ∗xzKyΓ 10Γ 1 ⊥ Γ 4Γ 2 ⊥ Γ 3K ∗w + KwFigure 49: (confer Figure 118) Simplicial cone K in R 2 and its dual K ∗drawn truncated. Conically independent generators Γ 1 and Γ 2 constituteextreme directions of K while Γ 3 and Γ 4 constitute extreme directions of K ∗ .Dotted ray-pairs bound translated cones K . Point x is comparable to pointz (and vice versa) but not to y ; z ≽ x ⇔ z − x ∈ K ⇔ z − x ≽ K0 iff ∃nonnegative coordinates for biorthogonal expansion of z − x . Point y is notcomparable to z because z does not belong to y ± K . Flipping a translatedcone is quite helpful for visualization: x ≼ z ⇔ x ∈ z − K ⇔ x − z ≼ K0.Points need not belong to K to be comparable; e.g., all points greater than wbelong to w + K .

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