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v2010.10.26 - Convex Optimization

v2010.10.26 - Convex Optimization

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A.5. EIGENVALUE DECOMPOSITION 619A.5.0.3eigenmatrixThe (right-)eigenvectors {s i } (1547) are naturally orthogonal w T i s j =0to left-eigenvectors {w i } except, for i=1... m , w T i s i =1 ; called abiorthogonality condition [365,2.2.4] [202] because neither set of left or righteigenvectors is necessarily an orthogonal set. Consequently, each dyad from adiagonalization is an independent (B.1.1) nonorthogonal projector becauses i w T i s i w T i = s i w T i (1556)(whereas the dyads of singular value decomposition are not inherentlyprojectors (confer (1563))).Dyads of eigenvalue decomposition can be termed eigenmatrices becauseSum of the eigenmatrices is the identity;A.5.1X s i w T i = λ i s i w T i (1557)m∑s i wi T = I (1558)i=1Symmetric matrix diagonalizationThe set of normal matrices is, precisely, that set of all real matrices havinga complete orthonormal set of eigenvectors; [393,8.1] [333, prob.10.2.31]id est, any matrix X for which XX T = X T X ; [159,7.1.3] [328, p.3]e.g., orthogonal and circulant matrices [168]. All normal matrices arediagonalizable.A symmetric matrix is a special normal matrix whose eigenvalues Λmust be real A.13 and whose eigenvectors S can be chosen to make a realorthonormal set; [333,6.4] [331, p.315] id est, for X ∈ S ms T1X = SΛS T = [s 1 · · · s m ] Λ⎣.⎡s T m⎤⎦ =m∑λ i s i s T i (1559)A.13 Proof. Suppose λ i is an eigenvalue corresponding to eigenvector s i of real A=A T .Then s H i As i= s T i As∗ i (by transposition) ⇒ s ∗Ti λ i s i = s T i λ∗ i s∗ i because (As i ) ∗ = (λ i s i ) ∗ byassumption. So we have λ i ‖s i ‖ 2 = λ ∗ i ‖s i‖ 2 . There is no converse.i=1

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