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v2009.01.01 - Convex Optimization

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536 CHAPTER 7. PROXIMITY PROBLEMS<br />

projection on the EDM cone, any solution acquired that way is necessarily<br />

suboptimal.<br />

A second recourse is problem redesign: A presupposition to all proximity<br />

problems in this chapter is that matrix H is given. We considered H having<br />

various properties such as nonnegativity, symmetry, hollowness, or lack<br />

thereof. It was assumed that if H did not already belong to the EDM cone,<br />

then we wanted an EDM closest to H in some sense; id est, input-matrix H<br />

was assumed corrupted somehow. For practical problems, it withstands<br />

reason that such a proximity problem could instead be reformulated so that<br />

some or all entries of H were unknown but bounded above and below by<br />

known limits; the norm objective is thereby eliminated as in the development<br />

beginning on page 287. That redesign (the art, p.8), in terms of the<br />

Gram-matrix bridge between point-list X and EDM D , at once encompasses<br />

proximity and completion problems.<br />

A third recourse is to apply the method of convex iteration just like we<br />

did in7.2.2.7.1. This technique is applicable to any semidefinite problem<br />

requiring a rank constraint; it places a regularization term in the objective<br />

that enforces the rank constraint.<br />

The importance and application of solving rank-constrained problems<br />

are enormous, a conclusion generally accepted gratis by the mathematics<br />

and engineering communities. For example, one might be interested in<br />

the minimal order dynamic output feedback which stabilizes a given linear<br />

time invariant plant (this problem is considered to be among the most<br />

important open problems in control). [227] Rank-constrained semidefinite<br />

programs arise in many vital feedback and control problems [146], optics<br />

[66], and communications [245] [219]. Rank-constrained problems also appear<br />

naturally in combinatorial optimization. (4.6.0.0.8)

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