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v2007.09.13 - Convex Optimization

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310 CHAPTER 5. EUCLIDEAN DISTANCE MATRIX(a)(c)x 3x 4 x 5x 5 x 6√d13√d14x 3ˇx 3 ˇx 4√d12ˇx 2x 1x 1x 2x 4x 1x 1Figure 78: (a) Given three distances indicated with absolute pointpositions ˇx 2 , ˇx 3 , ˇx 4 known and noncollinear, absolute position of x 1 in R 2can be precisely and uniquely determined by trilateration; solution to asystem of nonlinear equations. Dimensionless EDM graphs (b) (c) (d)represent EDMs in various states of completion. Line segments representknown absolute distances and may cross without vertex at intersection.(b) Four-point list must always be embeddable in affine subset havingdimension rankVN TDV N not exceeding 3. To determine relative position ofx 2 ,x 3 ,x 4 , triangle inequality is necessary and sufficient (5.14.1). Knowingall distance information, then (by injectivity of D (5.6)) point position x 1is uniquely determined to within an isometry in any dimension. (c) Whenfifth point is introduced, only distances to x 3 ,x 4 ,x 5 are required todetermine relative position of x 2 in R 2 . Graph represents first instanceof missing distance information; √ d 12 . (d) Three distances are absent( √ d 12 , √ d 13 , √ d 23 ) from complete set of interpoint distances, yet uniqueisometric reconstruction (5.4.2.2.5) of six points in R 2 is certain.(b)(d)

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