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v2007.11.26 - Convex Optimization

v2007.11.26 - Convex Optimization

v2007.11.26 - Convex Optimization

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A.4. SCHUR COMPLEMENT 515Origin of the term Schur complement is from complementary inertia:[77,2.4.4] Defineinertia ( G∈ S M) ∆ = {p,z,n} (1342)where p,z,n respectively represent number of positive, zero, and negativeeigenvalues of G ; id est,M = p + z + n (1343)Then, when A is invertible,and when C is invertible,inertia(G) = inertia(A) + inertia(C − B T A −1 B) (1344)inertia(G) = inertia(C) + inertia(A − BC −1 B T ) (1345)When A=C =0, denoting by σ(B)∈ R m + the nonincreasingly orderedsingular values of matrix B ∈ R m×m , then we have the eigenvalues[41,1.2, prob.17]([ ]) [ ]0 B σ(B)λ(G) = λB T =(1346)0 −Ξσ(B)andinertia(G) = inertia(B T B) + inertia(−B T B) (1347)where Ξ is the order-reversing permutation matrix defined in (1533).A.4.0.0.1 Example. Nonnegative polynomial. [27, p.163]Schur-form positive semidefiniteness is necessary and sufficient for quadraticpolynomial nonnegativity; videlicet, for all compatible x[x T 1] [ A bb T c] [ x1]≥ 0 ⇔ x T Ax + 2b T x + c ≥ 0 (1348)A.4.0.0.2 Example. Sparse Schur conditions.Setting matrix A to the identity simplifies the Schur conditions. Oneconsequence relates the definiteness of three quantities:[ ][ ]I BI 0B T ≽ 0 ⇔ C − B T B ≽ 0 ⇔C0 T C −B T ≽ 0 (1349)B

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