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v2009.01.01 - Convex Optimization

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Chapter 3<br />

Geometry of convex functions<br />

The link between convex sets and convex functions is via the<br />

epigraph: A function is convex if and only if its epigraph is a<br />

convex set.<br />

−Stephen Boyd & Lieven Vandenberghe [53,3.1.7]<br />

We limit our treatment of multidimensional functions 3.1 to finite-dimensional<br />

Euclidean space. Then an icon for a one-dimensional (real) convex function<br />

is bowl-shaped (Figure 67), whereas the concave icon is the inverted bowl;<br />

respectively characterized by a unique global minimum and maximum whose<br />

existence is assumed. Because of this simple relationship, usage of the term<br />

convexity is often implicitly inclusive of concavity in the literature. Despite<br />

the iconic imagery, the reader is reminded that the set of all convex, concave,<br />

quasiconvex, and quasiconcave functions contains the monotonic functions<br />

[177] [186,2.3.5]; e.g., [53,3.6, exer.3.46].<br />

3.1 vector- or matrix-valued functions including the real functions. Appendix D, with its<br />

tables of first- and second-order gradients, is the practical adjunct to this chapter.<br />

2001 Jon Dattorro. CO&EDG version 2009.01.01. All rights reserved.<br />

Citation: Jon Dattorro, <strong>Convex</strong> <strong>Optimization</strong> & Euclidean Distance Geometry,<br />

Meboo Publishing USA, 2005.<br />

195

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