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v2009.01.01 - Convex Optimization

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38 CHAPTER 2. CONVEX GEOMETRY<br />

(a)<br />

R<br />

(b)<br />

R 2<br />

(c)<br />

R 3<br />

Figure 12: (a) Ellipsoid in R is a line segment whose boundary comprises two<br />

points. Intersection of line with ellipsoid in R , (b) in R 2 , (c) in R 3 . Each<br />

ellipsoid illustrated has entire boundary constituted by zero-dimensional<br />

faces; in fact, by vertices (2.6.1.0.1). Intersection of line with boundary<br />

is a point at entry to interior. These same facts hold in higher dimension.<br />

2.1.7 classical boundary<br />

(confer2.6.1.3) Boundary of a set C is the closure of C less its interior<br />

presumed nonempty; [48,1.1]<br />

which follows from the fact<br />

∂ C = C \ int C (14)<br />

int C = C (15)<br />

assuming nonempty interior. 2.7 One implication is: an open set has a<br />

boundary defined although not contained in the set; interior of an open set<br />

equals the set itself.<br />

2.7 Otherwise, for x∈ R n as in (11), [222,2.1-2.3]<br />

the empty set is both open and closed.<br />

int{x} = ∅ = ∅

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