v2007.09.13 - Convex Optimization

538 APPENDIX C. SOME ANALYTICAL OPTIMAL RESULTSthe largest eigenvalue, and v N a normalized eigenvector correspondingto the smallest eigenvalue,v N = arg inf x T Ax (1468)‖x‖=1v 1 = arg sup x T Ax (1469)‖x‖=1For A∈ S N having eigenvalues λ(A)∈ R N , consider the unconstrainednonconvex optimization that is a projection on the rank-1 subset(2.9.2.1) of the boundary of positive semidefinite cone S N + : Definingλ 1 ∆ = maxi{λ(A) i } and corresponding eigenvector v 1minimizex‖xx T − A‖ 2 F = minimize tr(xx T (x T x) − 2Axx T + A T A)x{‖λ(A)‖ 2 , λ 1 ≤ 0=(1470)‖λ(A)‖ 2 − λ 2 1 , λ 1 > 0arg minimizex‖xx T − A‖ 2 F ={0 , λ1 ≤ 0v 1√λ1 , λ 1 > 0(1471)Proof. This is simply the Eckart & Young solution from7.1.2:x ⋆ x ⋆T ={0 , λ1 ≤ 0λ 1 v 1 v T 1 , λ 1 > 0(1472)minimizexGiven nonincreasingly ordered diagonalization A = QΛQ TΛ = δ(λ(A)) (A.5), then (1470) has minimum valuewhere⎧‖QΛQ T ‖ 2 F = ‖δ(Λ)‖2 , λ 1 ≤ 0⎪⎨⎛⎡‖xx T −A‖ 2 F =λ 1 Q ⎜⎢0⎝⎣. ..⎪⎩ ∥0⎤ ⎞ ∥ ∥∥∥∥∥∥2⎡⎥ ⎟⎦− Λ⎠Q T ⎢=⎣∥Fλ 10.0⎤⎥⎦− δ(Λ)∥2(1473), λ 1 > 0