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v2007.09.13 - Convex Optimization

v2007.09.13 - Convex Optimization

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496 APPENDIX A. LINEAR ALGEBRAA.3.1.0.4 Theorem. Positive (semi)definite principal submatrices. A.10A∈ S M is positive semidefinite if and only if all M principal submatricesof dimension M−1 are positive semidefinite and detA is nonnegative.A ∈ S M is positive definite if and only if any one principal submatrixof dimension M −1 is positive definite and detA is positive. ⋄If any one principal submatrix of dimension M−1 is not positive definite,conversely, then A can neither be. Regardless of symmetry, if A ∈ R M×M ispositive (semi)definite, then the determinant of each and every principalsubmatrix is (nonnegative) positive. [199,1.3.1]A.3.1.0.5[149, p.399]Corollary. Positive (semi)definite symmetric products.If A∈ S M is positive definite and any particular dimensionallycompatible matrix Z has no nullspace, then Z T AZ is positive definite.If matrix A∈ S M is positive (semi)definite then, for any matrix Z ofcompatible dimension, Z T AZ is positive semidefinite.A∈ S M is positive (semi)definite if and only if there exists a nonsingularZ such that Z T AZ is positive (semi)definite.If A ∈ S M is positive semidefinite and singular it remains possible, forsome skinny Z ∈ R M×N with N

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