1. Chapter 5. The Duality Theorem
Given an LP, we define another LP derived from the same data, but with different structure. This LP is called the dual problem (쌍대문제).
The main purpose to consider dual is to obtain an upper bound (estimate) on the optimal objective value of the given LP (for maximization problem) without solving it to optimality. Also dual problem provides optimality conditions of a solution 𝑥 ∗ for an LP and help to understand the behavior of the simplex method.
Very important concept to understand the properties of the LP and the simplex method.
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2. Preliminaries
Taking nonnegative linear combination of inequality constraints: Consider two constraints 𝑥 1 +2 𝑥 2 ≤3 and 2 𝑥 1 + 𝑥 2 ≤4 …. (1’)
In vector notation : 𝑎 1 ′𝑥≤3, 𝑎 2 ′𝑥≤4, where 𝑎 1 = , 𝑎 2 = 2 1
If we multiply scalars 𝑦 1 ≥0 to both sides of the 1st constraint and 𝑦 2 ≥0 to the 2nd constraint and add the l.h.s. and r.h.s. respectively,
we get 𝑦 1 +2 𝑦 2 𝑥 1 +(2 𝑦 1 + 𝑦 2 ) 𝑥 2 ≤(3 𝑦 1 +4 𝑦 2 ) ….. (2’)
In vector notation, 𝑦 1 𝑎 1 ′+ 𝑦 2 𝑎 2 ′ 𝑥≤(3 𝑦 1 +4 𝑦 2 )
Any vector that satisfies (1’) also satisfies (2’), but converse is not true.
Moreover, the coefficient vector in l.h.s. of (2’) is obtained by taking the nonnegative linear combination of the coefficient vectors in (1’)
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3. x2 2x1+x24 y1(1, 2)+y2(2, 1), y1, y2  0 x1+2x23 (1, 2) (2, 1)
(5/3, 2/3)
(y1a1’+y2a2’)x  (3y1+4y2) x1
(0, 0)
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4. Getting Dual Problem ex) max 4 𝑥 1 + 𝑥 2 +5 𝑥 3 +3 𝑥 4
s.t 𝑥 1 − 𝑥 2 − 𝑥 𝑥 4 ≤1
5 𝑥 𝑥 2 +3 𝑥 3 +8 𝑥 4 ≤55
− 𝑥 1 +2 𝑥 2 +3 𝑥 3 −5 𝑥 4 ≤3
𝑥 1 , 𝑥 2 , 𝑥 3 , 𝑥 4 ≥0
Lower bound on the optimal value :
consider feasible solution (0, 0, 1, 0)  𝑧 ∗ ≥5
(3, 0, 2, 0)  𝑧 ∗ ≥22
Upper bound : consider inequality obtained by multiplying 0 to the 1st , 1 to the 2nd, and 1 to the 3rd constraints and add the l.h.s. and r.h.s. respectively
4 𝑥 1 + 3𝑥 2 +6 𝑥 3 +3 𝑥 4 ≤ …… (1)
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5. Further, any feasible solution to the LP must satisfy
4 𝑥 1 + 3𝑥 2 +6 𝑥 3 +3 𝑥 4 ≤ …… (1)
Since we multiplied nonnegative numbers on both sides of the inequalities, any vector that satisfies the three constraints (which includes feasible solutions to the LP) also satisfies (1). Hence any feasible solution to LP, which satisfies the three constraints and nonnegativity, also satisfies (1).
Note that all the points satisfying 4 𝑥 1 +3 𝑥 2 +6 𝑥 3 +3 𝑥 4 ≤58 do not necessarily satisfy the three constraints in the LP.
Further, any feasible solution to the LP must satisfy
4 𝑥 1 + 𝑥 2 +5 𝑥 3 +3 𝑥 4 ≤4 𝑥 1 +3 𝑥 2 +6 𝑥 3 +3 𝑥 4 (  58)
since any feasible solution must satisfy nonnegativity constraints on the variables and the coefficients in the second expression is greater than or equal to the corresponding coefficients in the first expression (objective function).
So 58 is an upper bound on the optimal value of the LP.
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6. Now, we may use nonnegative weight 𝑦 𝑖 for 𝑖−𝑡ℎ constraint .
max 4 𝑥 1 + 𝑥 2 +5 𝑥 3 +3 𝑥 4
s.t 𝑥 1 − 𝑥 2 − 𝑥 𝑥 4 ≤1
5 𝑥 𝑥 2 +3 𝑥 3 +8 𝑥 4 ≤55
− 𝑥 1 +2 𝑥 2 +3 𝑥 3 −5 𝑥 4 ≤3
𝑥 1 , 𝑥 2 , 𝑥 3 , 𝑥 4 ≥0
Now, we may use nonnegative weight 𝑦 𝑖 for 𝑖−𝑡ℎ constraint .
𝑦 1 +5 𝑦 2 − 𝑦 3 𝑥 1 + − 𝑦 1 + 𝑦 2 +2 𝑦 3 𝑥 2 + − 𝑦 1 +3 𝑦 2 +3 𝑦 3 𝑥 𝑦 1 +8 𝑦 2 −5 𝑦 3 𝑥 4 ≤ 𝑦 𝑦 2 +3 𝑦 3
In vector notation,
𝑦 1 1,−1,−1,3 + 𝑦 2 5,1,3,8 + 𝑦 3 −1,2,3,−5 ′ 𝑥≤( 𝑦 𝑦 2 +3 𝑦 3 )
Objective function of the LP is 4 𝑥 1 + 𝑥 2 +5 𝑥 3 +3 𝑥 4
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7. Hence as long as the nonnegative weights 𝑦 𝑖 ′𝑠 satisfy
𝑦 1 +5 𝑦 2 − 𝑦 3 ≥4, − 𝑦 1 + 𝑦 2 +2 𝑦 3 ≥1
− 𝑦 1 +3 𝑦 2 +3 𝑦 3 ≥5, 3 𝑦 1 +8 𝑦 2 −5 𝑦 3 ≥3
we can use 𝑦 𝑦 2 +3 𝑦 3 as an upper bound on optimal value.
To find more accurate upper bound (smallest upper bound), we want to solve
Dual problem obtained. Note that the objective value of any feasible solution to the dual problem provides an upper bound on the optimal value of the given LP (called primal problem).
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8. P: primal problem, D: dual problem
General form
P: primal problem, D: dual problem
maximize 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 maximize 𝑐 ′ 𝑥
(P) subject to 𝑗=1 𝑛 𝑎 𝑖𝑗 𝑥 𝑗 ≤ 𝑏 𝑖 , 𝑖=1,…,𝑚 subject to 𝐴𝑥≤𝑏
𝑥 𝑗 ≥0, 𝑗=1,2,…,𝑛 𝑥≥0
minimize 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 minimize 𝑏 ′ 𝑦
(D) subject to 𝑖=1 𝑚 𝑎 𝑖𝑗 𝑦 𝑖 ≥ 𝑐 𝑗 , 𝑗=1,…,𝑛 subject to 𝑦 ′ 𝐴≥𝑐′
𝑦 𝑖 ≥0, 𝑖=1,2,…,𝑚 𝑦≥0
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9. Thm: (Weak duality relation)
Suppose ( 𝑥 1 ,…, 𝑥 𝑛 ) is a feasible solution to the primal problem (P) and ( 𝑦 1 ,…, 𝑦 𝑚 ) is a feasible solution to the dual problem (D), then
𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 ≤ 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 (5.4)
pf) 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 ≤ 𝑗=1 𝑛 ( 𝑖=1 𝑚 𝑎 𝑖𝑗 𝑦 𝑖 ) 𝑥 𝑗 = 𝑖=1 𝑚 ( 𝑗=1 𝑛 𝑎 𝑖𝑗 𝑥 𝑗 ) 𝑦 𝑖 ≤ 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 
Cor: If we can find a feasible 𝑥 ∗ to (P) and a feasible 𝑦 ∗ to (D) such that 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 ∗ = 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 ∗ , then x* is an optimal solution to (P) and y* is an optimal solution to (D).
pf) For all feasible solution 𝑥 to (P), we have
𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 ≤ 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 ∗ = 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 ∗ .
Similarly, for all feasible y to (D), we have
𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 ∗ = 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 ∗ ≤ 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 . 
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10. (i.e., no duality gap, dual optimal value – primal optimal value = 0)
Thm 5.1: [Strong Duality Theorem] If (P) has an optimal solution ( 𝑥 1 ∗ ,…, 𝑥 𝑛 ∗ ), then (D) also has an optimal solution, say ( 𝑦 1 ∗ ,…, 𝑦 𝑚 ∗ ), and
𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 ∗ = 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 ∗ .
(i.e., no duality gap, dual optimal value – primal optimal value = 0)
Note that strong duality theorem says that if (P) has an optimal solution, the dual (D) is neither unbounded nor infeasible, but always has an optimal solution.
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11. Idea of the proof: Read the optimal solution of the dual problem from the coefficients of the slack variables in the zeroth equation from the optimal dictionary (tableau).
ex)
Note that the dual variables 𝑦 1 , 𝑦 2 , 𝑦 3 matches naturally with slack variables 𝑥 5 , 𝑥 6 , 𝑥 7 . For example 𝑥 5 is slack variable for the first constraint and 𝑦 1 is dual variable for the first constraint, and so on.
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12. At optimality, the tableau looks
In the zeroth equation of the tableau, the coefficients of the slack variables are –11 for 𝑥 5 , 0 for 𝑥 6 , −6 for 𝑥 7 .
Assigning these values with reversed signs to the corresponding dual variables, we obtain desired optimal solution of the dual:
𝑦 1 =11, 𝑦 2 =0, 𝑦 3 =6.
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13. Suppose we performed row operations
Idea of the proof:
Note that the coefficients of 𝑥 5 , 𝑥 6 , and 𝑥 7 in the zeroth equation show what numbers we multiplied to the corresponding equation and add them to the zeroth equation in the elementary row operations (net effect of many row operations)
ex)
Suppose we performed row operations
(row2)  2(row 1) + (row 2), and then (row 0)  3(row 2) + (row 0).
The net effect in zeroth equation is adding 6(row 1) + 3(row 2) to the zeroth equation and the scalar we multiplied can be read from the coefficients of 𝑥 5 and 𝑥 6 in the zeroth equation.
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14. (ex-continued)
(row 1)2 +
(row 2)
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15. (ex-continued)
(row 2)  3 +
(row 0)
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16. Example – Initial tableau
Optimal tableau
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17. 𝑦 𝑖 ≤0, 𝑖=1,…,𝑚 (for slack variables)
Let 𝑦 𝑖 be the scalar we multiplied to the i-th equation and add to the zeroth equation in the net effect of many simplex iterations (elementary row operations).
Then the coefficients of slack variables in the zeroth equation represent the 𝑦 𝑖 values we multiplied to the i-th equation for 𝑖=1,…,𝑚.
Also the coefficients of structural variables in the zeroth equation are given as
𝑐 𝑗 + 𝑖=1 𝑚 𝑦 𝑖 𝑎 𝑖𝑗 , 𝑗=1,…,𝑛
Now in the optimal tableau, all the coefficients in the zeroth equation are  0, which implies
𝑦 𝑖 ≤0, 𝑖=1,…,𝑚 (for slack variables)
𝑐 𝑗 + 𝑖=1 𝑚 𝑦 𝑖 𝑎 𝑖𝑗 ≤0, 𝑗=1,…,𝑛 (for stuctural variables)
If we take (− 𝑦 𝑖 ), 𝑖=1,…,𝑚 as a dual solution, it is dual feasible.
(− 𝑦 𝑖 )≥0, 𝑖=1,…,𝑚
𝑖=1 𝑚 (− 𝑦 𝑖 ) 𝑎 𝑖𝑗 ≥ 𝑐 𝑗 , 𝑗=1,…,𝑛
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18. It is the idea of the proof.
Also the constant term in the zeroth equation gives the value 𝑖=1 𝑚 𝑦 𝑖 𝑏 𝑖 =− 𝑖=1 𝑚 (− 𝑦 𝑖 ) 𝑏 𝑖 .
So it is the negative of the dual objective value of the dual solution (− 𝑦 𝑖 ), 𝑖=1,…,𝑚.
Note that the constant term in the zeroth equation also gives the negative of the objective value of the current primal feasible solution.
So we have found a feasible dual solution giving the dual objective value which is same as the objective value of the current primal feasible solution.
From previous Corollary, the dual solution and the primal solution are optimal to the dual and the primal problem, respectively.
It is the idea of the proof.
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19. pf of strong duality theorem) Suppose we introduced slack variables
𝑥 𝑛+𝑖 = 𝑏 𝑖 − 𝑗=1 𝑛 𝑎 𝑖𝑗 𝑥 𝑗 (𝑖=1,…,𝑚)
and solve the LP by simplex and obtain optimal dictionary with
𝑧= 𝑧 ∗ + 𝑘=1 𝑛+𝑚 𝑐 𝑘 𝑥 𝑘 , 𝑐 𝑘 ≤0 ∀𝑘, 𝑧 ∗ = 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 ∗ .
Let 𝑦 𝑖 ∗ =− 𝑐 𝑛+𝑖 , 𝑖=1,…,𝑚.
We claim that 𝑦 𝑖 ∗ , 𝑖=1,…,𝑚 is an optimal dual solution, i.e. it satisfies dual constraints and 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 ∗ = 𝑧 ∗ .
Equate 𝑧= 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 and 𝑧= 𝑧 ∗ + 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 + 𝑗=𝑛+1 𝑛+𝑚 𝑐 𝑗 𝑥 𝑗
 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 = 𝑧 ∗ + 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 + 𝑗=𝑛+1 𝑛+𝑚 𝑐 𝑗 𝑥 𝑗 .
This equation must be satisfied by any 𝑥 that satisfies the dictionary (excluding the nonnegativity constraints) since the set of feasible solutions satisfying the equations does not change.
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20. Have 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 = 𝑧 ∗ + 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 + 𝑗=𝑛+1 𝑛+𝑚 𝑐 𝑗 𝑥 𝑗 .
Substitute 𝑥 𝑛+𝑖 = 𝑏 𝑖 − 𝑗=1 𝑛 𝑎 𝑖𝑗 𝑥 𝑗 , 𝑖=1,…,𝑚 into above.
Since any feasible solution to the dictionary should satisfy this.
 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 = 𝑧 ∗ + 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 − 𝑖=1 𝑚 𝑦 𝑖 ∗ ( 𝑏 𝑖 − 𝑗=1 𝑛 𝑎 𝑖𝑗 𝑥 𝑗 )
 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 = 𝑧 ∗ − 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 ∗ + 𝑗=1 𝑛 ( 𝑐 𝑗 + 𝑖=1 𝑚 𝑎 𝑖𝑗 𝑦 𝑖 ∗ ) 𝑥 𝑗
Now this equation should hold for all feasible solutions to the dictionary (disregarding nonnegativity constraints). From the initial dictionary, we know that any feasible solution to the dictionary (disregarding nonnegativity constraints) can be obtained by assigning arbitrary values to 𝑥 1 ,…, 𝑥 𝑛 and setting 𝑥 𝑛+𝑖 = 𝑏 𝑖 − 𝑗=1 𝑛 𝑎 𝑖𝑗 𝑥 𝑗 , (𝑖=1,…,𝑚).
Use these solutions. Note that, in the above equation, the variables 𝑥 𝑛+𝑖 ′𝑠 do not appear. So it must hold for any value of 𝑥 𝑗 , 𝑗=1,…,𝑛.
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21. Equality must hold for all values of 𝑥 1 ,…, 𝑥 𝑛 .
𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 = 𝑧 ∗ − 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 ∗ + 𝑗=1 𝑛 ( 𝑐 𝑗 + 𝑖=1 𝑚 𝑎 𝑖𝑗 𝑦 𝑖 ∗ ) 𝑥 𝑗
Equality must hold for all values of 𝑥 1 ,…, 𝑥 𝑛 .
Hence 𝑧 ∗ = 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 ∗ , 𝑐 𝑗 = 𝑐 𝑗 + 𝑖=1 𝑚 𝑎 𝑖𝑗 𝑦 𝑖 ∗ , 𝑗=1,…,𝑛
𝑐 𝑘 ≤0 ∀𝑘  𝑐 𝑗 − 𝑖=1 𝑚 𝑎 𝑖𝑗 𝑦 𝑖 ∗ ≤0  𝑖=1 𝑚 𝑎 𝑖𝑗 𝑦 𝑖 ∗ ≥ 𝑐 𝑗 , 𝑗=1,…,𝑛.
− 𝑦 𝑖 ∗ ≤0  𝑦 𝑖 ∗ ≥0, 𝑖=1,…,𝑚
Hence 𝑦 ∗ is dual feasible.
Also we have that 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 ∗ = 𝑧 ∗ = 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 ∗ .
Since 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 ∗ = 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 ∗ ≤ 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 for all dual feasible 𝑦 (weak duality), 𝑦 𝑖 ∗ , 𝑖=1,…,𝑚 is an optimal dual solution and 𝑖=1 𝑚 𝑏 𝑖 𝑦 𝑖 ∗ = 𝑗=1 𝑛 𝑐 𝑗 𝑥 𝑗 ∗ 
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