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1. What Is Optimization?

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1. What Is Optimization? 1. WHAT IS OPTIMIZATION? Optimization problem: Maximizing or minimizing some function relative to some set, often representing a range of choices available in a certain situation. The function allows comparison of the different choices for determining which might be “best.” Common applications: Minimal cost, maximal profit, minimal error, optimal design, optimal management, variational principles. Goals of the subject: The understanding of Modeling issues— What to look for in setting up an optimization problem? What features are advantageous or disadvantageous? What devices/tricks of formulation are available? How can problems usefully be categorized? Analysis of solutions— What is meant by a “solution?” When do solutions exist, and when are they unique? How can solutions be recognized and characterized? What happens to solutions under perturbations? Numerical methods— How can solutions be determined by iterative schemes of computation? What modes of local simplification of a problem are convenient/appropriate? How can different solution techniques be compared and evaluated? Distinguishing features of optimization as a mathematical discipline: descriptive −→ prescriptive equations −→ inequalities linear/nonlinear −→ convex/nonconvex differential calculus −→ subdifferential calculus 1 Finite-dimensional optimization: The case where a choice corresponds to selecting the values of a finite number of real variables, called decision variables. For general purposes the decision variables may be denoted by x1, . , xn and each possible choice n therefore identified with a point x = (x1, . , xn) in the space IR . This is what we’ll be focusing on in this course. n Feasible set: The subset C of IR representing the allowable choices x = (x1, . , xn). Objective function: The function f0(x) = f0(x1, . , xn) that is to be maximized or minimized over C. Constraints: Side conditions that are used to specify the feasible set C within IRn. n Equality constraints: Conditions of the form fi(x) = ci for certain functions fi on IR n and constants ci in IR . Inequality constraints: Conditions of the form fi(x) ≤ ci or fi(x) ≥ ci for certain n functions fi on IR and constants ci in IR. Range constraints: Conditions restricting the values of some decision variables to lie within certain closed intervals of IR. Very important in many situations, for instance, are nonnegativity constraints: some variables xj may only be allowed to take values ≥ 0; the interval then is [0, ∞). Range constraints can also arise from the desire to keep a variable between certain upper and lower bounds. Linear constraints: Range constraints or conditions of the form fi(x) = ci, fi(x) ≤ ci, or fi(x) ≥ ci, in which the function is linear in the standard sense of being expressible as sum of constant coefficients times the variables x1, . , xn. Data parameters: General problem statements usually involve not only decision vari- ables but symbols designating known coefficients, constants, or other data ele- ments. Conditions on such elements, such as the nonnegativity of a particular coefficient, are not among the “constraints” in a problem of optimization, since the numbers in question are supposed to be given and aren’t subject to choice. Mathematical programming: A traditional synonym for finite-dimensional optimiza- tion. This usage predates “computer programming,” which actually arose from early attempts at solving optimization problems on computers. “Programming,” with the meaning of optimization, survives in problem classifications such as linear program- ming, quadratic programming, convex programming, integer programming, etc. 2 EXAMPLE 1: Engineering Design General description. In the design of some object, system or structure, the values of certain parameters can be chosen subject to some conditions expressing their ranges and interrelationships. The choice determines the values of a number of other variables on which the desirability of the end product depends, such as cost, weight, speed, bandwidth, reliability,... Among the choices of the design parameters that meet certain performance specifications, which is the “best” by some criterion? Particular case: optimal proportions of a can. A cylindrical can of a given volume V0 is to be proportioned in such a way as to minimize the total cost of the material in a box of 12 cans, arranged in a 3 × 4 pattern. The cost expression takes the form c1S1 + c2S2, where S1 is the surface area of the 12 cans and S2 is the surface area of the box. (The coefficients c1 and c2 are positive.) A side requirement is that no dimension of the box can exceed a given amount D0. design parameters: r = radius of can, h = height of can 2 volume constraint: πr h = V0 (or ≥ V0, see below!) 2 surface area of cans: S1 = 12(2πr + 2πrh) = 24πr(r + h) box dimensions: 8r × 6r × h 2 surface area of box: S2 = 2(48r + 8rh + 6rh) = 4r(24r + 7h) size constraints: 8r ≤ D0, 6r ≤ D0, h ≤ D0 nonnegativity constraints: r ≥ 0, h ≥ 0 (!) Summary. The design choices that are available can be identified with the set C consisting of all the pairs (r, h) ∈ IR2 that satisfy the conditions 2 r ≥ 0, h ≥ 0, 8r ≤ D0, 6r ≤ D0, h ≤ D0, πr h = V0. Over this set we wish to minimize the function 2 f0(r, h) = c1 24πr(r + h) + c2 4r(24r + 7h) = d1r + d2rh, where d1 = 24πc1 + 96c2 and d2 = 24πc1 + 28c2. 3 Comments. This example illustrates several features that are quite typically found in problems of optimization. Redundant constraints: It is obvious that the condition 6r ≤ D0 is implied by the other constraints and therefore could be dropped without affecting the prob- lem. But in problems with many variables and constraints such redundancy may be hard to recognize. From a practical point of view, the elimination of redundant constraints could pose a challenge as serious as that of solving the optimization problem itself. Inactive constraints: It could well be true that the optimal pair (r, h) (unique??) is such that either the condition 8r ≤ D0 or the condition h ≤ D0 is satisfied as a strict inequality, or both. In that case the constraints in question are inactive in the local characterization of optimal point, although they do affect the shape of the set C. Again, however, there is little hope, in a problem with many variables and constraints, of determining by some preliminary procedure just which constraints will be active and which will not. This is the crux of the difficulty in many numerical approaches. 2 Redundant variables: It would be possible to solve the equation πr h = V0 for h in terms of r and thereby reduce the given problem to one in terms of just r, rather than (r, h). Fine—but besides being a technique that is usable only in special circumstances, the elimination of variables from (generally nonlinear) systems of equations is not necessarily helpful. There may be a trade-off between the lower dimensionality achieved in this way and other properties. 2 Inequalities versus equations: The constraint πr h = V0 could be written in the 2 form πr h ≥ V0 without affecting anything about the solution. This is because of the nature of the cost function; no pair (r, h) in the larger set C0, obtained by substituting this weaker condition for the equation, can minimize f0 unless actually (r, h) ∈ C. While it may seem instinctive to prefer the equation to the inequality in the formulation, the inequality turns to be superior in the present case because the set C0 happens to be “convex,” whereas C isn’t. Convexity: This problem is not fully of “convex” type in itself, despite the pre- ceding remark. Nonetheless, it can be made convex by a certain change of variables, as will be seen later. The lesson is that the formulation of a prob- lem of optimization can be quite subtle, when it comes to bringing out crucial features like convexity. 4 EXAMPLE 2: Management of Systems General description. A sequence of decisions must be made in discrete time which will affect the operation of some kind of “system,” often of an economic nature. The decisions, each in terms of choosing the values of a number of variables, have to respect various limitations in resources. Typically the desire is to minimize cost, or maximize profit or efficiency, say, over a certain time horizon. Particular case: an inventory model. A warehouse with total capacity a (in units of volume) is to be operated over time periods t = 1,...,T as the sole facility for the supply of a number of different commodities (or medicines, or equipment parts, etc.), indexed by j = 1, . , n. The demand for commodity j during period t is the known amount dtj ≥ 0 (in volume units)—this is a deterministic approach to modeling the situation. In each period t it is possible not only to fill demands but to acquire additional supplies up to certain limits, so as to maintain stocks. The problem is to plan the pattern of acquiring supplies in such a way as to maximize the net profit over the T periods, relative to the original inventory amounts and the desired terminal inventory amounts. inventory variables: xtj units of j at the end of period t Pn inventory constraints: xtj ≥ 0, j=1 xtj ≤ a for t = 1,...,T initial inventory: x0j units of j given at the beginning terminal constraints: xT j = bj (given amounts) for j = 1, . , n inventory costs: stj dollars per unit of j held from t to t + 1 supply variables: utj units of j acquired during period j supply constraints: 0 ≤ utj ≤ atj (given availabilities) supply costs: ctj dollars per unit of j acquired during t dynamical constraints: xtj = max 0, xt−1,j + utj − dtj rewards: ptj dollars per unit of filled demand filled demand: min dtj, xt−1,j + utj units of j during period t PT Pn net profit: t=1 j=1 ptj min dtj, xt−1,j + utj − stjxtj − ctjutj Summary.
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