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v2007.09.13 - Convex Optimization

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220 CHAPTER 3. GEOMETRY OF CONVEX FUNCTIONSFigure 61: Iconic unimodal differentiable quasiconvex function of twovariables graphed in R 2 × R on some open disc in R 2 . Note reversal ofcurvature in direction of gradient.3.2.3.0.6 Exercise. log det function.Show by two different methods: log detX is concave on the interior of thepositive semidefinite cone.3.3 QuasiconvexQuasiconvex functions [46,3.4] [147] [245] [277] [177,2] are useful inpractical problem solving because they are unimodal (by definition whennonmonotonic); a global minimum is guaranteed to exist over any convex setin the function domain; e.g., Figure 61.3.3.0.0.1 Definition. Quasiconvex function.f(X) : R p×k →R is a quasiconvex function of matrix X iff domf is a convexset and for each and every Y,Z ∈domf , 0≤µ≤1f(µ Y + (1 − µ)Z) ≤ max{f(Y ), f(Z)} (538)

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