chap06_8up

Heath 10 74 denitions existence and uniqueness

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Unformatted text preview: x) ∂xi ∂xj which is symmetric For continuously differentiable f : S ⊆ Rn → R, any interior point x∗ of S at which f has local minimum must be critical point of f Michael T. Heath 10 / 74 Deﬁnitions Existence and Uniqueness Optimality Conditions Second-Order Optimality Condition For function of one variable, one can ﬁnd extremum by differentiating function and setting derivative to zero Optimization Problems One-Dimensional Optimization Multi-Dimensional Optimization Scientiﬁc Computing f (x∗ ) = L(x, λ) = Its Hessian is given by T Jg (x∗ )λ HL (x, λ) = where Jg is Jacobian matrix of g , and λ is vector of Lagrange multipliers Michael T. Heath Scientiﬁc Computing T B (x, λ) Jg (x) Jg (x) O where This condition says we cannot reduce objective function without violating constraints Optimization Problems One-Dimensional Optimization Multi-Dimensional Optimization T f (x) + Jg (x)λ g (x) m B (x, λ) = Hf (x) + λi Hgi (x) i=1 13 / 74 Deﬁnitions Existence and Uniqueness Optimality Conditions Michael T. Heath Optimization Problems One-Dimensional Optimization Multi-Dimensional Optimization Constrained Optimality, continued Scientiﬁc Computing 14 / 74 Deﬁnitions Existence and Uniqueness Optimality Conditions Constrained Optimality, continued Together, necessary condition and feasibility imply critical point of Lagrangian function, L(x, λ) = T f (x) + Jg (x)λ =0 g (x) If inequalities are present, then KKT optimality conditions also require nonnegativity of Lagrange multipliers corresponding to inequalities, and complementarity condition Hessian of Lagrangian is symmetric, but not positive deﬁnite, so critical point of L is saddle point rather than minimum or maximum Critical point (x∗ , λ∗ ) of L is constrained minimum of f if B (x∗ , λ∗ ) is positive deﬁnite on null space of Jg (x∗ ) If columns of Z form basis for null space, then test projected Hessian Z T BZ for positive deﬁniteness Michael T. Heath Scientiﬁc Computing 15 / 74 Michael T. Heath Scientiﬁc Computing 16 / 74 Optimization Problems One-Dimensional Optimization Multi-Dimensional Optimization Optimization Problems One-Dimensional Optimization Multi-Dimensional Optimization Deﬁnitions Existence and Uniqueness Optimality Conditions Golden Section Search Successive Parabolic Interpolation Newton’s Method Unimodality Sensitivity and Conditioning Function minimization and equation solving are closely related problems, but their sensitivities differ For minimizing function of one variable, we need “bracket” for solution analogous to sign change for nonlinear equation In one dimension, absolute condition number of root x∗ of equation f (x) = 0 is 1/|f (x∗ )|, so if |f (ˆ)| ≤ , then x |x − x∗ | may be as large as /|f (x∗ )| ˆ Real-valued function f is unimodal on interval [a, b] if there is unique x∗ ∈ [a, b] such that f (x∗ ) is minimum of f on [a, b], and f is strictly decreasing for x ≤ x∗ , strictly increasing for x∗ ≤ x For minimizing f , Taylor series expansion f (ˆ) = f (x∗ + h) x = f (x∗ ) + f (x∗ )h + 1 f (x∗ )h2 + O(h3 ) 2 shows that, since f (x∗ ) = 0, i...
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