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v2010.10.26 - Convex Optimization

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v2010.10.26 - Convex Optimization

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A.4. SCHUR COMPLEMENT 615From Corollary A.3.1.0.3 eigenvalues are related by0 ≼ λ(C −B T A −1 B) ≼ λ(C) (1532)0 ≼ λ(A − BC −1 B T ) ≼ λ(A) (1533)which meansrank(C −B T A −1 B) ≤ rankC (1534)rank(A − BC −1 B T ) ≤ rankA (1535)Therefore[ A BrankB T C]≤ rankA + rankC (1536)A.4.0.1.4 Lemma. Rank of Schur-form block. [136] [134]Matrix B ∈ R m×n has rankB≤ ρ if and only if there exist matrices A∈ S mand C ∈ S n such that[ ][ ]A 0A Brank0 T ≤ 2ρ and G =CB T ≽ 0 (1537)C⋄Schur-form positive semidefiniteness alone implies rankA ≥ rankB andrankC ≥ rankB . But, even in absence of semidefiniteness, we must alwayshave rankG ≥ rankA, rankB, rankC by fundamental linear algebra.A.4.1Determinant[ ] A BG =B T C(1538)We consider again a matrix G partitioned like (1511), but not necessarilypositive (semi)definite, where A and C are symmetric.When A is invertible,When C is invertible,detG = detA det(C − B T A −1 B) (1539)detG = detC det(A − BC −1 B T ) (1540)

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