1 1 Slide © 2000 South-Western College Publishing/ITP Slides Prepared by JOHN LOUCKS

2 2 Slide Chapter 5 Linear Programming: The Simplex Method n An Overview of the Simplex Method n Standard Form n Tableau Form n Setting Up the Initial Simplex Tableau n Improving the Solution n Calculating the Next Tableau n Solving a Minimization Problem n Special Cases

3 3 Slide Overview of the Simplex Method n Steps Leading to the Simplex Method FormulateProblem As LP FormulateProblem Put In StandardForm StandardForm TableauForm TableauFormExecuteSimplexMethodExecuteSimplexMethod

4 4 Slide Example: Initial Formulation n A Minimization Problem MIN z = 2 x x x 3 MIN z = 2 x x x 3 s. t. x 1 + x 2 + x 3 < 30 s. t. x 1 + x 2 + x 3 < 30 2 x 1 + x x 3 > 60 2 x 1 + x x 3 > 60 x 1 - x x 3 = 20 x 1 - x x 3 = 20 x j > 0 j = 1,2,3 x j > 0 j = 1,2,3

5 5 Slide Standard Form n Putting an LP formulation into standard form involves adding slack and/or subtracting surplus variables, as discussed in Chapter 2.

6 6 Slide Example: Standard Form n Problem in Standard Form MIN z = 2 x x x 3 s. t. x 1 + x 2 + x 3 < 30 s. t. x 1 + x 2 + x 3 < 30 2 x 1 + x x 3 > 60 2 x 1 + x x 3 > 60 x 1 - x x 3 = 20 x 1 - x x 3 = 20 x j > 0 j = 1,2,3 x j > 0 j = 1,2,3

7 7 Slide Tableau Form n A set of equations is in tableau form if for each equation: its right hand side (RHS) is non-negative, andits right hand side (RHS) is non-negative, and there is a basic variable. (A basic variable for an equation is a variable whose coefficient in the equation is +1 and whose coefficient in all other equations of the problem is 0.)there is a basic variable. (A basic variable for an equation is a variable whose coefficient in the equation is +1 and whose coefficient in all other equations of the problem is 0.)

8 8 Slide Example: Tableau Form n Problem in Tableau Form MIN z = 2 x x x 3 +0 s 1 -0 s 2 + Ma 2 + Ma 3 s. t. x 1 + x 2 + x 3 + s 1 = 30 s. t. x 1 + x 2 + x 3 + s 1 = 30 2 x 1 + x x 3 - s 2 + a 2 = 60 2 x 1 + x x 3 - s 2 + a 2 = 60 x 1 - x x 3 + a 3 = 20 x 1 - x x 3 + a 3 = 20 x j > 0 j = 1,2,3 and s j, a j > 0 j = 1,2 x j > 0 j = 1,2,3 and s j, a j > 0 j = 1,2

9 9 Slide Simplex Tableau n The simplex tableau is a convenient means for performing the calculations required by the simplex method.

10 Slide Setting Up Initial Simplex Tableau n Step 1: If the problem is a minimization problem, multiply the objective function by -1. n Step 2: If the problem formulation contains any constraints with negative right-hand sides, multiply each constraint by -1. n Step 3: Add a slack variable to each < constraint. n Step 4: Subtract a surplus variable and add an artificial variable to each > constraint.

11 Slide Setting Up Initial Simplex Tableau n Step 5: Add an artificial variable to each = constraint. n Step 6: Set each slack and surplus variable's coefficient in the objective function equal to zero. n Step 7: Set each artificial variable's coefficient in the objective function equal to - M, where M is a very large number. n Step 8: Each slack and artificial variable becomes one of the basic variables in the initial basic feasible solution.

12 Slide Simplex Method n Step 1: Determine Entering Variable Identify the variable with the most positive value in the c j - z j row. (The entering column is called the pivot column.)Identify the variable with the most positive value in the c j - z j row. (The entering column is called the pivot column.) n Step 2: Determine Leaving Variable For each positive number in the entering column, compute the ratio of the right-hand side values divided by these entering column values.For each positive number in the entering column, compute the ratio of the right-hand side values divided by these entering column values. If there are no positive values in the entering column, STOP; the problem is unbounded.If there are no positive values in the entering column, STOP; the problem is unbounded. Otherwise, select the variable with the minimal ratio. (The leaving row is called the pivot row.)Otherwise, select the variable with the minimal ratio. (The leaving row is called the pivot row.)

13 Slide Simplex Method n Step 3: Generate Next Tableau Divide the pivot row by the pivot element (the entry at the intersection of the pivot row and pivot column) to get a new row. We denote this new row as (row *).Divide the pivot row by the pivot element (the entry at the intersection of the pivot row and pivot column) to get a new row. We denote this new row as (row *). Replace each non-pivot row i with:Replace each non-pivot row i with: [new row i ] = [current row i ] - [( a ij ) x (row *)], [new row i ] = [current row i ] - [( a ij ) x (row *)], where a ij is the value in entering column j of row i where a ij is the value in entering column j of row i n Step 4: Calculate z j Row for New Tableau For each column j, multiply the objective function coefficients of the basic variables by the corresponding numbers in column j and sum them.For each column j, multiply the objective function coefficients of the basic variables by the corresponding numbers in column j and sum them.

14 Slide Simplex Method n Step 5: Calculate c j - z j Row for New Tableau For each column j, subtract the z j row from the c j row in the tableau.For each column j, subtract the z j row from the c j row in the tableau. If none of the values in the c j - z j row are positive, GO TO STEP 1.If none of the values in the c j - z j row are positive, GO TO STEP 1. If there is an artificial variable in the basis with a positive value, the problem is infeasible. STOP.If there is an artificial variable in the basis with a positive value, the problem is infeasible. STOP. Otherwise, an optimal solution has been found. The current values of the basic variables are optimal. The optimal values of the non-basic variables are all zero.Otherwise, an optimal solution has been found. The current values of the basic variables are optimal. The optimal values of the non-basic variables are all zero. If any non-basic variable's c j - z j value is 0, alternate optimal solutions might exist. STOP.If any non-basic variable's c j - z j value is 0, alternate optimal solutions might exist. STOP.

15 Slide Example: Simplex Method n Solve the following problem by the simplex method: Max 12 x x x 3 Max 12 x x x 3 s.t. 2 x x x 3 < 50 s.t. 2 x x x 3 < 50 x 1 - x 2 - x 3 > 0 x 1 - x 2 - x 3 > 0 x x 3 > 0 x x 3 > 0 x 1, x 2, x 3 > 0 x 1, x 2, x 3 > 0

16 Slide Example: Simplex Method n Writing the Problem in Standard Form We can avoid introducing artificial variables to the second and third constraints by multiplying each by -1 (making them < constraints). Thus, slack variables s 1, s 2, and s 3 are added to the three constraints, respectively. Max 12 x x x s s s 3 Max 12 x x x s s s 3 s.t. 2 x x x 3 + s 1 < 50 s.t. 2 x x x 3 + s 1 < 50 x 1 - x 2 - x 3 + s 2 > 0 x 1 - x 2 - x 3 + s 2 > 0 x x 3 + s 3 > 0 x x 3 + s 3 > 0 x 1, x 2, x 3, s 1, s 2, s 3 > 0 x 1, x 2, x 3, s 1, s 2, s 3 > 0

17 Slide Example: Simplex Method n Initial Simplex Tableau x 1 x 2 x 3 s 1 s 2 s 3 x 1 x 2 x 3 s 1 s 2 s 3 Basis c B Basis c B s s s (* row) s (* row) s s z j z j c j - z j c j - z j

18 Slide Example: Simplex Method n Simplex Method (Iteration 1) Step 1: Determine the Entering VariableStep 1: Determine the Entering Variable The most positive c j - z j = 18. Thus x 2 is the entering variable. Step 2: Determine the Leaving VariableStep 2: Determine the Leaving Variable Take the ratio between the right hand side and positive numbers in the x 2 column: 50/3 = 16 2/3 50/3 = 16 2/3 0/1 = 0 minimum 0/1 = 0 minimum s 2 is the leaving variable and the 1 is the pivot element. s 2 is the leaving variable and the 1 is the pivot element.

19 Slide Example: Simplex Method n Simplex Method (Iteration 1) Step 3: Generate New TableauStep 3: Generate New Tableau Divide the second row by 1, the pivot element. Call the "new" (in this case, unchanged) row the "* row". Divide the second row by 1, the pivot element. Call the "new" (in this case, unchanged) row the "* row". Subtract 3 x (* row) from row 1. Subtract 3 x (* row) from row 1. Subtract -1 x (* row) from row 3. Subtract -1 x (* row) from row 3. New rows 1, 2, and 3 are shown in the upcoming tableau. New rows 1, 2, and 3 are shown in the upcoming tableau.

20 Slide Example: Simplex Method n Simplex Method (Iteration 1) Step 4: Calculate z j Row for New TableauStep 4: Calculate z j Row for New Tableau The new z j row values are obtained by multiplying the c B column by each column, element by element and summing. For example, z 1 = 5(0) + -1(18) + -1(0) = -18. For example, z 1 = 5(0) + -1(18) + -1(0) = -18.

21 Slide Example: Simplex Method n Simplex Method (Iteration 1) Step 5: Calculate c j - z j Row for New TableauStep 5: Calculate c j - z j Row for New Tableau The new c j - z j row values are obtained by subtracting z j value in a column from the c j value in the same column. For example, c 1 - z 1 = 12 - (-18) = 30.

22 Slide Example: Simplex Method n Simplex Method (Iteration 1) New TableauNew Tableau x 1 x 2 x 3 s 1 s 2 s 3 x 1 x 2 x 3 s 1 s 2 s 3 Basis c B Basis c B s (* row) s (* row) x x s s z j z j c j - z j c j - z j

23 Slide Example: Simplex Method n Simplex Method (Iteration 2) Step 1: Determine the Entering VariableStep 1: Determine the Entering Variable The most positive c j - z j = 30. x 1 is the entering variable. Step 2: Determine the Leaving VariableStep 2: Determine the Leaving Variable Take the ratio between the right hand side and positive numbers in the x 1 column: 10/5 = 2 minimum 10/5 = 2 minimum There are no ratios for the second and third rows because their column elements (-1) are negative. There are no ratios for the second and third rows because their column elements (-1) are negative. Thus, s 1 (corresponding to row 1) is the leaving Thus, s 1 (corresponding to row 1) is the leaving variable and 5 is the pivot element. variable and 5 is the pivot element.

24 Slide Example: Simplex Method n Simplex Method (Iteration 2) Step 3: Generate New TableauStep 3: Generate New Tableau Divide row 1 by 5, the pivot element. (Call this new row 1 the "* row"). Subtract (-1) x (* row) from the second row. Subtract (-1) x (* row) from the second row. Subtract (-1) x (* row) from the third row. Subtract (-1) x (* row) from the third row. Step 4: Calculate z j Row for New TableauStep 4: Calculate z j Row for New Tableau The new z j row values are obtained by multiplying the c B column by each column, element by element and summing. The new z j row values are obtained by multiplying the c B column by each column, element by element and summing. For example, z 3 =.2(12) + 1.2(18) +.2(0) = 24. For example, z 3 =.2(12) + 1.2(18) +.2(0) = 24.

25 Slide Example: Simplex Method n Simplex Method (Iteration 2) Step 5: Calculate c j - z j Row for New TableauStep 5: Calculate c j - z j Row for New Tableau The new c j - z j row values are obtained by subtracting z j value in a column from the c j value in the same column. For example, c 3 -z 3 = 10 - (24) = -14. For example, c 3 -z 3 = 10 - (24) = -14. Since there are no positive numbers in the c j - z j row, this tableau is optimal. The optimal solution is: x 1 = 10; x 2 = 10; x 3 = 0; s 1 = 0; s 2 = 0 s 3 = 10, and the optimal value of the objective function is 300. Since there are no positive numbers in the c j - z j row, this tableau is optimal. The optimal solution is: x 1 = 10; x 2 = 10; x 3 = 0; s 1 = 0; s 2 = 0 s 3 = 10, and the optimal value of the objective function is 300.

26 Slide Example: Simplex Method n Simplex Method (Iteration 2) New TableauNew Tableau x 1 x 2 x 3 s 1 s 2 s 3 x 1 x 2 x 3 s 1 s 2 s 3 Basis c B Basis c B x (* row) x (* row) x x s s z j z j c j - z j c j - z j

27 Slide Infeasibility n Infeasibility is detected in the simplex method when an artificial variable remains positive in the final tableau.

28 Slide Example: Infeasibility MAX 2 x x 2 MAX 2 x x 2 s. t. 4 x x 2 < 12 s. t. 4 x x 2 < 12 2 x 1 + x 2 > 8 2 x 1 + x 2 > 8 x 1, x 2 > 0 x 1, x 2 > 0

29 Slide Example: Infeasibility x 1 x 2 s 1 s 2 a 2 x 1 x 2 s 1 s 2 a 2 BasisC B M BasisC B M x /4 1/4 003 a 2 - M 0 -1/2 -1/ z j 2 (1/2) M (1/2) M M - M -2 M +6 +3/2 +1/2 c j - z j 0 -(1/2) M -(1/2) M - M 0 +9/2 -1/2 c j - z j 0 -(1/2) M -(1/2) M - M 0 +9/2 -1/2 This is the last tableau since all c j - z j < 0. But an artificial variable is still positive, so the problem is infeasible.

30 Slide Unboundedness n A linear program has an unbounded solution if all entries in an entering column are non-positive.

31 Slide Example: Unboundedness MAX 2 x x 2 s. t. 4 x x 2 > 12 s. t. 4 x x 2 > 12 2 x 1 + x 2 > 8 2 x 1 + x 2 > 8 x 1, x 2 > 0 x 1, x 2 > 0

32 Slide Example: Unboundedness x 1 x 2 s 1 s 2 Basis c B x s z j c j - z j c j - z j Note that c 4 - z 4 = 4 (positive), but its column is all non- positive. This indicates that the problem is unbounded.

33 Slide Alternative Optimal Solutions n A linear program has alternate optimal solutions if the final tableau has a c j - z j value equal to 0 for a non-basic variable.

34 Slide Alternative Optimal Solutions n Consider the following optimal tableau: x 1 x 2 x 3 s 1 s 2 s 3 s 4 Basis c B s s x x x x s s z j c j - z j

35 Slide Alternative Optimal Solutions In the previous slide we see that the optimal solution is: x 1 = 4, x 2 = 6, x 3 = 0, and z = 32 Note that x 3 is non-basic and its c 3 - z 3 = 0. This 0 indicates that if x 3 were increased, the value of the objective function would not change. Another optimal solution can be found by choosing x 3 as the entering variable and performing one iteration of the simplex method. The new tableau on the next slide shows an alternative optimal solution is: x 1 = 7, x 2 = 0, x 3 = 3, and z = 32

36 Slide Alternative Optimal Solutions x 1 x 2 x 3 s 1 s 2 s 3 s 4 Basis c B s s x /211 -1/200 3 x /211 -1/200 3 x /202 3/200 7 x /202 3/200 7 s /202 5/201 9 s /202 5/201 9 z j c j - z j

37 Slide Degeneracy n A degenerate solution to a linear program is one in which at least one of the basic variables equals 0. This can occur at formulation or if there is a tie for the minimizing value in the ratio test to determine the leaving variable.

38 Slide The End of Chapter 5