STEVENS - Hillier_10e_ch04.pdf - Chapter 4 Solving Linear...

Unformatted text preview: Chapter 4 Solving Linear Programming Problems: The Simplex Method Dr. David R. Nowicki Professor Fall 2018 2 4.1 The Essence of the Simplex Method • Algebraic procedure – Underlying concepts are geometric • Revisit Wyndor example – Figure 4.1 shows constraint boundary lines • Points of intersection are corner-point solutions • Five points on corners of feasible region are CPF solutions • Adjacent CPF solutions – Share a constraint boundary 3 The Essence of the Simplex Method 4 The Essence of the Simplex Method • Optimality test – If a CPF solution has no adjacent CPF solution that is better (as measured by Z): • It must be an optimal solution • Solving the example with the simplex method – Choose an initial CPF solution (0,0) and decide if it is optimal – Move to a better adjacent CPF solution – Iterate until an optimal solution is found 5 4.2 Setting Up the Simplex Method • First step: convert functional inequality constraints into equality constraints – Done by introducing slack variables – Resulting form known as augmented form – Example: constraint 1 ≤ 4 is equivalent to 1 + 3 = 4 and 3 ≥ 0 6 Setting Up the Simplex Method 7 Setting Up the Simplex Method • Augmented solution – Solution for the original decision variables augmented by the slack variables • Basic solution – Augmented corner-point solution • Basic feasible (BF) solution – Augmented CPF solution 8 Setting Up the Simplex Method • Properties of a basic solution – Each variable designated basic or nonbasic – Number of basic variables equals number of functional constraints – The nonbasic variables are set equal to zero – Values of basic variables obtained as simultaneous solution of system of equations – If basic variables satisfy nonnegativity constraints, basic solution is a BF solution 9 4.3 The Algebra of the Simplex Method 0 4.4 The Simplex Method in Tabular Form • Tabular form – Records only the essential information: • Coefficients of the variables • Constants on the right-hand sides of the equations • The basic variable appearing in each equation – Example shown in Table 4.3 on next slide 1 The Simplex Method in Tabular Form 2 The Simplex Method in Tabular Form • Summary of the simplex method – Initialization • Introduce slack variables – Optimality test • Optimal if and only if every coefficient in row 0 is nonnegative – Iterate (if necessary) to obtain the next BF solution • Determine entering and leaving basic variables • Minimum ratio test 3 4.5 Tie Breaking in the Simplex Method • Tie for the entering basic variable – Decision may be made arbitrarily • Tie for the leaving basic variable – Matters theoretically but rarely in practice – Choose arbitrarily • Condition of no leaving basic variable – Z is unbounded – Indicates a mistake has been made 4 Tie Breaking in the Simplex Method • Multiple optimal solutions – Simplex method stops after one optimal BF solution is found – Often other optimal solutions exist and would be meaningful choices – Method exists to detect and find other optimal BF solutions 5 4.6 Adapting to Other Model Forms • Simplex method adjustments – Needed when problem is not in standard form – Made during initialization step • Artificial-variable technique – Dummy variable introduced into each constraint that needs one – Becomes initial basic variable for that equation 6 Adapting to Other Model Forms • Types of nonstandard forms – Equality constraints – Negative right-hand sides – Functional constraints in greater-than-or-equal-to form – Minimizing Z • Solving the radiation therapy problem – Text reviews two methods: Big M and two-phase 7 Adapting to Other Model Forms • No feasible solutions – Constructing an artificial feasible solution may lead to a false optimal solution – Artificial-variable technique provides a way to indicate whether this is the case • Variables are allowed to be negative – Example: negative value indicates a decrease in production rate – Negative values may have a bound or no bound 8 4.7 Postoptimality Analysis • Simplex method role 9 Postoptimality Analysis • Reoptimization – Alternative to solving the problem again with small changes – Involves deducing how changes in the model get carried along to the final simplex tableau – Optimal solution for the revised model: • Will be much closer to the prior optimal solution than to an initial BF solution constructed the usual way 0 Postoptimality Analysis • Shadow price – Measures the marginal value of resource i – The rate at which Z would increase if more of the resource could be made available – Given by the coefficient of the ith slack variable in row 0 of the final simplex tableau 1 Postoptimality Analysis • Sensitivity analysis – Purpose: to identify the sensitive parameters • These must be estimated with special care – Can be done graphically if there are just two variables – Can be performed in Microsoft Excel 2 Postoptimality Analysis 3 Postoptimality Analysis • Parametric linear programming – Study of how the optimal solution changes as many of the parameters change simultaneously over some range – Used for investigation of trade-offs in parameter values – Technique presented in Section 8.2 4 4.8 Computer Implementation • Simplex method ideally suited for execution on a computer • Computer code for the simplex method – Widely available for all modern systems – Follows the revised simplex method • Main factor determining time to solution – Number of functional constraints • Rule of thumb: number of iterations required equals twice the number of functional constraints 5 4.9 The Interior-Point Approach to Solving Linear Programming Problems • Alternative to the simplex method developed in the 1980s – Far more complicated • Uses an iterative approach starting with a feasible trial solution – Trial solutions are interior points • Inside the boundary of the feasible region • Advantage: large problems do not require many more iterations than small problems 6 The Interior-Point Approach to Solving Linear Programming Problems 7 The Interior-Point Approach to Solving Linear Programming Problems • Disadvantage – Limited capability for performing a postoptimality analysis • Approach: switch over to simplex method 8 4.10 Conclusions • Simplex method – Efficient and reliable approach for solving linear programming problems – Algebraic procedure – Efficiently performs postoptimality analysis – Moves from current BF solution to a better BF solution – Best performed by computer except for the very simplest problems ...
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