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v2007.09.13 - Convex Optimization

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510 APPENDIX A. LINEAR ALGEBRAA.7 ZerosA.7.1zero normFor any given norm, by definition,A.7.20 entry‖x‖ l= 0 ⇔ x = 0 (1363)If a positive semidefinite matrix A = [A ij ] ∈ R n×n has a 0 entry A ii on itsmain diagonal, then A ij + A ji = 0 ∀j . [199,1.3.1]Any symmetric positive semidefinite matrix having a 0 entry on its maindiagonal must be 0 along the entire row and column to which that 0 entrybelongs. [109,4.2.8] [149,7.1, prob.2]A.7.3 0 eigenvalues theoremThis theorem is simple, powerful, and widely applicable:A.7.3.0.1 Theorem. Number of 0 eigenvalues.For any matrix A∈ R m×nrank(A) + dim N(A) = n (1364)by conservation of dimension. [149,0.4.4]For any square matrix A∈ R m×m , the number of 0 eigenvalues is at leastequal to dim N(A)dim N(A) ≤ number of 0 eigenvalues ≤ m (1365)while the eigenvectors corresponding to those 0 eigenvalues belong to N(A).[247,5.1] A.16A.16 We take as given the well-known fact that the number of 0 eigenvalues cannot be lessthan the dimension of the nullspace. We offer an example of the converse:⎡ ⎤1 0 1 0A = ⎢ 0 0 1 0⎥⎣ 0 0 0 0 ⎦1 0 0 0

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