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v2009.01.01 - Convex Optimization

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434 CHAPTER 5. EUCLIDEAN DISTANCE MATRIX<br />

Secondly, the second half (1054b) of the alternation takes place in a<br />

different vector space; S N h (versus S N−1 ). From5.6 we know these two<br />

vector spaces are related by an isomorphism, S N−1 =V N (S N h ) (923), but not<br />

by an isometry.<br />

We have, therefore, no guarantee from theory of alternating projection<br />

that the alternation (1054) converges to a point, in the set of all<br />

EDMs corresponding to affine dimension not in excess of 3, belonging to<br />

dvec EDM N ∩ Π T K M+ .<br />

5.13.2.4 Interlude<br />

We have not implemented the second half (1057) of alternation (1054) for<br />

USA map data because memory-demands exceed the capability of our 32-bit<br />

laptop computer.<br />

5.13.2.4.1 Exercise. Convergence of isotonic solution by alternation.<br />

Empirically demonstrate convergence, discussed in5.13.2.3, on a smaller<br />

data set.<br />

<br />

It would be remiss not to mention another method of solution to this<br />

isotonic reconstruction problem: Once again we assume only comparative<br />

distance data like (1046) is available. Given known set of indices I<br />

minimize rankV DV<br />

D<br />

(1060)<br />

subject to d ij ≤ d kl ≤ d mn ∀(i,j,k,l,m,n)∈ I<br />

D ∈ EDM N<br />

this problem minimizes affine dimension while finding an EDM whose<br />

entries satisfy known comparative relationships. Suitable rank heuristics<br />

are discussed in4.4.1 and7.2.2 that will transform this to a convex<br />

optimization problem.<br />

Using contemporary computers, even with a rank heuristic in place of the<br />

objective function, this problem formulation is more difficult to compute than<br />

the relaxed counterpart problem (1053). That is because there exist efficient<br />

algorithms to compute a selected few eigenvalues and eigenvectors from a<br />

very large matrix. Regardless, it is important to recognize: the optimal<br />

solution set for this problem (1060) is practically always different from the<br />

optimal solution set for its counterpart, problem (1052).

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