# A NEW LOOK AT CONVEX ANALYSIS AND OPTIMIZATION

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1 1 A NEW LOOK AT CONVEX ANALYSIS AND OPTIMIZATION Dimitri Bertsekas M.I.T. FEBRUARY 2003

2 2 OUTLINE Convexity issues in optimization Historical remarks Our treatment of the subject Three unifying lines of analysis Common geometrical framework for duality and minimax Unifying framework for existence of solutions and duality gap analysis Unification of Lagrange multiplier theory using an enhanced Fritz John theory and the notion of pseudonormality

3 3 WHY IS CONVEXITY IMPORTANT IN OPTIMIZATION I A convex function has no local minima that are not global A nonconvex function can be convexified while maintaining the optimality of its minima A convex set has nonempty relative interior A convex set has feasible directions at any point f(x) Epigraph f(x) Epigraph Convex set Feasible Directions Nonconvex set x x Convex function Nonconvex function

4 WHY IS CONVEXITY IMPORTANT IN OPTIMIZATION II The existence of minima of convex functions is conveniently characterized using directions of recession A polyhedral convex set is characterized by its extreme points and extreme directions 4 v 1 P 0 Recession Cone R f v 2 Level Sets of Convex Function f v 3

5 5 WHY IS CONVEXITY IMPORTANT IN OPTIMIZATION III A convex function is continuous and has nice differentiability properties Convex functions arise prominently in duality Convex, lower semicontinuous functions are self-dual with respect to conjugacy f(z) (x, f(x)) (-d, 1) z

6 6 SOME HISTORY Late 19th-Early 20th Century: Caratheodory, Minkowski, Steinitz, Farkas 40s-50s: The big turning point Game Theory: von Neumann Duality: Fenchel, Gale, Kuhn, Tucker Optimization-related convexity: Fenchel 60s-70s: Consolidation Rockafellar 80s-90s: Extensions to nonconvex optimization and nonsmooth analysis Clarke, Mordukovich, Rockafellar-Wets

7 7 ABOUT THE BOOK Convex Analysis and Optimization, by D. P. Bertsekas, with A. Nedic and A. Ozdaglar (March 2003) Aims to make the subject accessible through unification and geometric visualization Unification is achieved through several new lines of analysis

8 8 NEW LINES OF ANALYSIS I A unified geometrical approach to convex programming duality and minimax theory Basis: Duality between two elementary geometrical problems II A unified view of theory of existence of solutions and absence of duality gap Basis: Reduction to basic questions on intersections of closed sets IIIA unified view of theory of existence of Lagrange multipliers/constraint qualifications Basis: The notion of constraint pseudonormality, motivated by a new set of enhanced Fritz John conditions

9 9 OUTLINE OF THE BOOK Chapter 1 Basic Convexity Concepts Convex Sets and Functions Semicontinuity and Epigraphs Convex and Affine Hulls Relative Interior Directions of Recession Closed Set Intersections Chapters 2-4 Existence of Optimal Solutions Min Common/ Max Crossing Duality Minimax Saddle Point Theory Polyhedral Convexity Representation Thms Extreme Points Linear Programming Integer Programming Subdifferentiability Calculus of Subgradients Conical Approximations Optimality Conditions Chapters 5-8 Constrained Optimization Duality Geometric Multipliers Strong Duality Results Fenchel Duality Conjugacy Exact Penalty Functions Lagrange Multiplier Theory Pseudonormality Constraint Qualifications Exact Penalty Functions Fritz John Theory Dual Methods Subgradient and Incremental Methods

10 10 OUTLINE OF DUALITY ANALYSIS Closed Set Intersection Results Min Common/ Max Crossing Theory Enhanced Fritz John Conditions Preservation of Closedness Under Partial Minimization p(u) = inf x F(x,u) Minimax Theory Nonlinear Farkas Lemma Absence of Duality Gap Fenchel Duality Theory Existence of Geometric Multipliers Existence of Informative Geometric Multipliers

11 11 I MIN COMMON/MAX CROSSING DUALITY

12 12 GEOMETRICAL VIEW OF DUALITY w Min Common Point w * _ M w Min Common Point w * M M 0 Max Crossing Point q * u Max Crossing Point q * 0 u (a) (b) Min Common Point w * w _ M Max Crossing Point q * S M 0 u (c)

13 13 APPROACH Prove theorems about the geometry of M Conditions on M guarantee that w* = q* Conditions on M that guarantee existence of a max crossing hyperplane Choose M to reduce the constrained optimization problem, the minimax problem, and others, to special cases of the min common/max crossing framework Specialize the min common/max crossing theorems to duality and minimax theorems

14 14 CONVEX PROGRAMMING DUALITY Primal problem: min f(x) subject to x X and g j (x) 0, j=1,,r Dual problem: max q(µ) subject to µ 0 where the dual function is q(µ) = inf x L(x,µ) = inf x {f(x) + µ g(x)} Optimal primal value = inf x sup L(x,µ) µ 0 Optimal dual value = sup µ 0 inf x L(x,µ) Min common/max crossing framework: M = epi(p), p(u) = inf x X, gj(x) uj f(x)

15 15 VISUALIZATION (µ,1) M {(g(x),f(x)) x X} q * f* p(u) q(µ) = inf u {p(u) + µ u} u

16 16 MINIMAX / ZERO SUM GAME THEORY ISSUES Given a function Φ(x,z), where x X and z Z, under what conditions do we have inf x sup z Φ(x,z) = sup z inf x Φ(x,z) Assume convexity/concavity, semicontinuity of Φ Min common/max crossing framework: M = epigraph of the function p(u) = inf x sup z {Φ(x,z) - u z} inf x sup z Φ = Min common value sup z inf x Φ = Max crossing value

17 17 VISUALIZATION w w inf x sup z φ(x,z) (µ,1) M = epi(p) = min common value w * M = epi(p) (µ,1) inf x sup z φ(x,z) 0 q(µ) = min common value w * u sup z inf x φ(x,z) sup z inf x φ(x,z) = max crossing value q * 0 q(µ) u = max crossing value q * (a) (b)

18 18 TWO ISSUES IN CONVEX PROGRAMMING AND MINIMAX When is there no duality gap ( in convex programming), or inf sup = sup inf (in minimax)? When does an optimal dual solution exist (in convex programming), or the sup is attained (in minimax)? Min common/max crossing framework shows that 1st question is a lower semicontinuity issue 2nd question is an issue of existence of a nonvertical support hyperplane (or subgradient) at the origin Further analysis is needed for more specific answers

19 19 II UNIFICATION OF EXISTENCE AND NO DUALITY GAP ISSUES

20 20 INTERSECTIONS OF NESTED FAMILIES OF CLOSED SETS Two basic problems in convex optimization Attainment of a minimum of a function f over a set X Existence of a duality gap The 1st question is a set intersection issue: The set of minima is the intersection of the nonempty level sets {x X f(x) γ} The 2nd question is a lower semicontinuity issue: When is the function p(u) = inf x F(x,u) lower semicontinuous, assuming F(x,u) is convex and lower semicontinuous?

21 21 PRESERVATION OF SEMICONTINUITY UNDER PARTIAL MINIMIZATION 2nd question also involves set intersection Key observation: For p(u) = inf x F(x,u), we have Closure(P(epi(F))) epi(p) P(epi(F)) where P(. ) is projection on the space of u. So if projection preserves closedness, F is l.s.c. C C k+1 x C k y y k+1 y k P(C) Given C, when is P(C) closed? If y k is a sequence in P(C) that converges to y, - we must show that the intersection of the C k is nonempty

22 22 UNIFIED TREATMENT OF EXISTENCE OF SOLUTIONS AND DUALITY GAP ISSUES Results on nonemptiness of intersection of a nested family of closed sets No duality gap results In convex programming inf sup Φ = sup inf Φ Existence of minima of f over X

23 23 THE ROLE OF POLYHEDRAL ASSUMPTIONS: AN EXAMPLE Polyhedral Constraint Set X Level Sets of Convex Function f Nonpolyhedral Constraint Set X Level Sets of Convex Function f x 2 x 2 X X 0 x 1 0 x 1 Minimum Attained f(x) = e x 1 Minimum not Attained The min is attained if X is polyhedral, and f is constant along common directions of recession of f and X

24 24 THE ROLE OF QUADRATIC FUNCTIONS Results on nonemptiness of intersection of sets defined by quadratic inequalities If f is bounded below over X, the min of f over X is attained If the optimal value is finite, there is no duality gap Linear programming Quadratic programming Semidefinite programming

25 25 III LAGRANGE MULTIPLIER THEORY / PSEUDONORMALITY

26 26 LAGRANGE MULTIPLIERS Problem (smooth, nonconvex): Min f(x) subject to x X, h i (x)=0, i = 1,,m Necessary condition for optimality of x* (case X = R n ): Under some constraint qualification, we have f(x*) + Σ I λ i h i (x*) = 0 for some Lagrange multipliers λ i Basic analytical issue: What is the fundamental structure of the constraint set that guarantees the existence of a Lagrange multiplier? Standard constraint qualifications (case X = R n ): The gradients h i (x*) are linearly independent The functions h i are affine

27 27 ENHANCED FRITZ JOHN CONDITIONS If x* is optimal, there exist µ 0 0 and λ i, not all 0, such that µ 0 f(x*) + Σ i λ i h i (x*) = 0, and a sequence {x k } with x k x* and such that f (xk ) < f(x*) for all k, λ i h i (x k ) > 0 for all i with λ i 0 and all k NOTE: If µ 0 > 0, the λ i are Lagrange multipliers with a sensitivity property (we call them informative)

28 28 PSEUDONORMALITY Definition: A feasible point x* is pseudonormal if one cannot find λ i and a sequence {x k } with x k x* such that Σ i λ i h i (x*) = 0, Σ i λ i h i (x k ) > 0 for all k Pseudonormality at x* guarantees that, if x* is optimal, we can take µ 0 = 1 in the F-J conditions (so there exists an informative Lagrange multiplier) u 2 u 2 0 λ Tε u 1 λ 0 Tε u 1 Map an ε -ball around x* onto the constraint space T ε = {h(x) x-x* < ε} Pseudonormal h i : Affine Not Pseudonormal

29 29 INFORMATIVE LAGRANGE MULTIPLIERS The Lagrange multipliers obtained from the enhanced Fritz John conditions have a special sensitivity property: They indicate the constraints to violate in order to improve the cost We call such multipliers informative Proposition: If there exists at least one Lagrange multiplier vector, there exists one that is informative

30 30 EXTENSIONS/CONNECTIONS TO NONSMOOTH ANALAYSIS F-J conditions for an additional constraint x X The stationarity condition becomes - ( f(x*) + Σ I λ i h i (x*)) (normal cone of X at x*) X is called regular at x* if the normal cone is equal to the polar of its tangent cone at x* (example: X convex) If X is not regular at x*, the Lagrangian may have negative slope along some feasible directions Regularity is the fault line beyond which there is no satisfactory Lagrange multiplier theory

31 31 THE STRUCTURE OF THE THEORY FOR X = R n Independent Constraint Gradients Linear Constraints Mangasarian Fromovitz Condition Pseudonormality Existence of Lagrange Multipliers Existence of Informative Multipliers

32 32 THE STRUCTURE OF THE THEORY FOR X: REGULAR New Mangasarian Fromovitz-Like Condition Slater Condition Pseudonormality Existence of Lagrange Multipliers Existence of Informative Multipliers

33 33 EXTENSIONS Enhanced Fritz John conditions and pseudonormality for convex problems, when existence of a primal optimal solution is not assumed Connection of pseudonormality and exact penalty functions Connection of pseudonormality and the classical notion of quasiregularity

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