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v2007.09.13 - Convex Optimization

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532 APPENDIX B. SIMPLE MATRICESFigure 115: Gimbal: a mechanism imparting three degrees of dimensionalfreedom to a Euclidean body suspended at the device’s center. Each ring isfree to rotate about one axis. (Courtesy of The MathWorks Inc.)Reflection matrices have eigenvalues equal to ±1 and so detQ=±1. Itis natural to expect a relationship between reflection and projection matricesbecause all projection matrices have eigenvalues belonging to {0, 1}. Infact, any reflection matrix Q is related to some orthogonal projector P by[151,1, prob.44]Q = I − 2P (1444)Yet P is, generally, neither orthogonal or invertible. (E.3.2)λ(Q) ∈ R n , |λ(Q)| = 1 (1445)Reflection is with respect to R(P ) ⊥ . Matrix 2P −I represents antireflection.Every orthogonal matrix can be expressed as the product of a rotation anda reflection. The collection of all orthogonal matrices of particular dimensiondoes not form a convex set.B.5.3Rotation of range and rowspaceGiven orthogonal matrix Q , column vectors of a matrix X are simultaneouslyrotated by the product QX . In three dimensions (X ∈ R 3×N ), the precisemeaning of rotation is best illustrated in Figure 115 where the gimbal aidsvisualization of rotation achievable about the origin.

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