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v2007.09.13 - Convex Optimization

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176 CHAPTER 2. CONVEX GEOMETRYαα ≥ β ≥ γβCx ⋆ −∇f(x ⋆ )γ{z | f(z) = α}{y | ∇f(x ⋆ ) T (y − x ⋆ ) = 0, f(x ⋆ )=γ}Figure 53: Shown is a plausible contour plot in R 2 of some arbitrarydifferentiable real convex function f(x) at selected levels α , β , and γ ;id est, contours of equal level f (level sets) drawn (dashed) in function’sdomain. Function is minimized over convex set C at point x ⋆ iff negativegradient −∇f(x ⋆ ) belongs to normal cone to C there. In circumstancedepicted, normal cone is a ray whose direction is coincident with negativegradient. From results in3.1.9 (p.211), ∇f(x ⋆ ) is normal to the γ-sublevelset by Definition E.9.1.0.1.

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