chap06_8up

# E f x must be large whenever x is large for some

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Unformatted text preview: widely separated starting points and see if all produce same result x →∞ i.e., f (x) must be large whenever x is large For some problems, such as linear programming, global optimization is more tractable Michael T. Heath Michael T. Heath Optimization Problems One-Dimensional Optimization Multi-Dimensional Optimization Scientiﬁc Computing If f is coercive on closed, unbounded set S ⊆ Rn , then f has global minimum on S 7 / 74 Michael T. Heath Scientiﬁc Computing 8 / 74 Optimization Problems One-Dimensional Optimization Multi-Dimensional Optimization Optimization Problems One-Dimensional Optimization Multi-Dimensional Optimization Deﬁnitions Existence and Uniqueness Optimality Conditions Level Sets Deﬁnitions Existence and Uniqueness Optimality Conditions Uniqueness of Minimum Level set for function f : S ⊆ Rn → R is set of all points in S for which f has some given constant value Set S ⊆ Rn is convex if it contains line segment between any two of its points For given γ ∈ R, sublevel set is Function f : S ⊆ Rn → R is convex on convex set S if its graph along any line segment in S lies on or below chord connecting function values at endpoints of segment Lγ = {x ∈ S : f (x) ≤ γ } If continuous function f on S ⊆ Rn has nonempty sublevel set that is closed and bounded, then f has global minimum on S Any local minimum of convex function f on convex set S ⊆ Rn is global minimum of f on S Any local minimum of strictly convex function f on convex set S ⊆ Rn is unique global minimum of f on S If S is unbounded, then f is coercive on S if, and only if, all of its sublevel sets are bounded Scientiﬁc Computing Michael T. Heath Optimization Problems One-Dimensional Optimization Multi-Dimensional Optimization 9 / 74 Michael T. Heath Deﬁnitions Existence and Uniqueness Optimality Conditions Optimization Problems One-Dimensional Optimization Multi-Dimensional Optimization First-Order Optimality Condition For twice continuously differentiable f : S ⊆ Rn → R, we can distinguish among critical points by considering Hessian matrix Hf (x) deﬁned by Generalization to function of n variables is to ﬁnd critical point, i.e., solution of nonlinear system {Hf (x)}ij = f (x) = 0 where f (x) is gradient vector of f , whose ith component is ∂f (x)/∂xi At critical point x∗ , if Hf (x∗ ) is positive deﬁnite, then x∗ is minimum of f negative deﬁnite, then x∗ is maximum of f indeﬁnite, then x∗ is saddle point of f singular, then various pathological situations are possible But not all critical points are minima: they can also be maxima or saddle points Scientiﬁc Computing Michael T. Heath 11 / 74 Optimization Problems One-Dimensional Optimization Multi-Dimensional Optimization Deﬁnitions Existence and Uniqueness Optimality Conditions Constrained Optimality Scientiﬁc Computing 12 / 74 Deﬁnitions Existence and Uniqueness Optimality Conditions Constrained Optimality, continued If problem is constrained, only feasible directions are relevant Lagrangian function L : Rn+m → R, is deﬁned by L(x, λ) = f (x) + λT g (x) For equality-constrained problem Its gradient is given by min f (x) subject to g (x) = 0 Rn Rn Rm , where f : → R and g : → with m ≤ n, necessary condition for feasible point x∗ to be solution is that negative gradient of f lie in space spanned by constraint normals, − ∂ 2 f (...
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