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v2010.10.26 - Convex Optimization

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210 CHAPTER 2. CONVEX GEOMETRYK = { [ X 1 X 2 · · · X M ] d | d ≽ 0 } (474)by the extremes theorem (2.8.1.1.1). Defining X [X 1 X 2 · · · X M ] (withany redundant [sic] columns optionally removed from X), then K ∗ can beexpressed ((363), Cone Table S p.195)K ∗ = {y | X T y ≽ 0} =M⋂{y | Xi T y ≽ 0} =i=1M⋂K ∗ i (475)i=1To find the extreme directions of the dual cone, first we observe that somefacets of each simplicial part K i are common to facets of K by assumption,and the union of all those common facets comprises the set of all facets ofK by design. For any particular proper polyhedral cone K , the extremedirections of dual cone K ∗ are respectively orthogonal to the facets of K .(2.13.6.1) Then the extreme directions of the dual cone can be foundamong inward-normals to facets of the component simplicial cones K i ; thosenormals are extreme directions of the dual simplicial cones K ∗ i . From thetheorem and Cone Table S (p.195),K ∗ =M⋂K ∗ i =i=1M⋂i=1{X †Ti c | c ≽ 0} (476)The set of extreme directions {Γ ∗ i } for proper dual cone K ∗ is thereforeconstituted by those conically independent generators, from the columnsof all the dual simplicial matrices {X †Ti } , that do not violate discretedefinition (363) of K ∗ ;{ }Γ ∗ 1 , Γ ∗ 2 ... Γ ∗ N{}= c.i. X †Ti (:,j), i=1... M , j =1... n | X † i (j,:)Γ l ≥ 0, l =1... N(477)where c.i. denotes selection of only the conically independent vectors fromthe argument set, argument (:,j) denotes the j th column while (j,:) denotesthe j th row, and {Γ l } constitutes the extreme directions of K . Figure 50b(p.143) shows a cone and its dual found via this algorithm.

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