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v2009.01.01 - Convex Optimization

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646 APPENDIX E. PROJECTION<br />

T<br />

x<br />

T ⊥ R(Q)<br />

cone(Q)<br />

cone(U)<br />

Px<br />

Figure 138: Nonorthogonal projection of x∈ R 3 on R(U)= R 2 under<br />

biorthogonality condition; id est, Px=UQ T x such that Q T U =I . Any<br />

point along imaginary line T connecting x to Px will be projected<br />

nonorthogonally on Px with respect to horizontal plane constituting<br />

R 2 = aff cone(U) in this example. Extreme directions of cone(U) correspond<br />

to two columns of U ; likewise for cone(Q). For purpose of illustration, we<br />

truncate each conic hull by truncating coefficients of conic combination at<br />

unity. Conic hull cone(Q) is headed upward at an angle, out of plane of<br />

page. Nonorthogonal projection would fail were N(Q T ) in R(U) (were T<br />

parallel to a line in R(U)).

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