Nonlinear Programming Models

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Nonlinear Programming
min (max) f (x)
subject to
(1a)
hi(x) = 0,
i = 1,...,p
(1b)
gi(x) ≤ 0,
i = 1,...,m
(1c)
where
x = (x1, ..., xn)
and
f, h1, ..., hp, g1, ..., gm
are continuous functions
Feasible Solution: Any point x satisfying (1b) – (1c).
Optimal Solution: A feasible –x such that f(–x) ≤ f(x) for all x
feasible.
Box Size Problem
Let x, y and z be respectively the length, width and height
of a box.
Problem: Select the box dimensions to minimize the
surface area subject to a constraint on volume.
minimize f(x,y,z) = 2xy + 2xz + 2yz
subject to xyz = V
x ≥ 0, y ≥ 0, z ≥ 0
Reliability Problem
Let ri = probability that component i will not fail
ci = cost of component i
wi = weight of component i
xi = number of redundant (parallel) components i
Reliability of component i: Ri = 1 – (1 – ri)
(1+xi)
 The reliability of a series system is the product of the
component type reliabilities.
Problem: Find the optimal number of redundant
components of each type, subject to constraints on
maximum total cost, C, and weight, W.
n
Maximize
 [1 – (1 – ri)
(1+xi)
i=1
subject to
n
 c ix i ≤ C
i=1
n
 wixi ≤ W
i=1
xi ≥ 0, i = 1,…,n
]
Example 1
min f(x1, x2) = (x1 – 3)2 + (x2 – 2)2
subject to
2
g1(x1, x2) = x1 – x2 – 3 ≤ 0
g2(x1, x2) = x2 – 1 ≤ 0
g3(x1, x2) = x1 ≥ 0
Isovalue Contour: (x1 – 3)2 + (x2 – 2)2 = c
(circle with radius c and center at (3, 2))
Geometric Solution for Example 1
g3
(3, 2)
Optimal
solution
(2, 1)
g2
•
•
Isovalue
contours
x1
g1
Feasible
region
x2
Example 2
min f(x1, x2) = |x1 – 2| + |x2 – 2|
subject to
2
2
h1(x1, x2) = x1 + x2 – 1 = 0
2
g1(x1, x2) = x1 – x2 ≥ 0
4
x2
f= 2
3
(2, 2)
•
2
Optimum
1
–2
•
1
–1
Isovalue
contours
f= 1
2
3
4
–1
h1= 0
–2
g1 •
0
5
x1
Convex Functions
Let x = (x1,...,xn). A function f(x) is convex if and only if
f (x1 + (1 – )x2) ≤ f (x1) + (1 – )f (x2)
for all  such that 0 ≤  ≤ 1.
A function f(x) is strictly convex if the inequality is strict
except when  = 0 or 1.
A function f (x) is concave if and only if
f (x1 + (1 – )x2) ≥ f (x1) + (1 – )f (x2).
If f (x) is convex then –f (x) is concave.
f(x)
 f(x1 ) +(1- )f(x 2)
f( x + (1- )x 2 )
1
x1
 x 1+ (1- )x 2
x2
x
Convex Sets
A feasible region, C, is a convex set if any point on
a straight line connecting any two feasible points,
x1, x2  C, is also feasible.
Mathematically, this property is expressed as
x = x1 + (1–)x2  C
for all  such that 0 ≤  ≤ 1.
When g(x) is convex, the set
{x  Rn : g(x) ≤ b}
defines a convex region.
g(x)
b
0
x1
x2
x
Convex vs. Nonconvex Programming
minimize f(x)
subject to gi(x) ≤ bi,
i = 1,…,m
 When f(x) is convex and each gi(x) is convex,
any local optimum is a global optimum.
 When either f(x) is nonconvex or any gi(x) is
nonconvex, it is possible that a local optimum
may not be a global optimum.
 Most nonlinear programming algorithms are
search techniques that find local optima.
Properties of Linear Programs
(a) The set of feasible solutions {x  n : Ax = b, x ≥ 0} is
convex.
(b) The set of all n-tuples (x1,...,xn) which yields a specified
value of the objective function is a hyperplane.
(c) A local extremum is also a global extremum.
(d) If the optimal value of the objective function is finite, at
least one extreme point in the feasible region will be
optimal.
Difficulties with Nonlinearities
When nonlinearities are present in the model, none of the above
properties are likely to hold.
Illustration of Local Optima
max f(x1, x2) = 25(x1 – 2)2 + (x2 – 2)2
subject to
x1 + x2 ≥ 2
–x1 + x2 ≤ 2
x1 + x2 ≤ 6
x1 – 3x2 ≤ 2
x1, x2 ≥ 0
x2
(2, 4)
•
4
f = 100
3
Feasible
region
f= 4
2
•
f = 226
• (5, 1)
1
•
0
1
2
3
4
5
6
x
1
More Local Extrema
Determining Convexity
Single Dimension
• f(x) is convex if and only if
d2f(x)
≥ 0 for all x.
dx2
• A function f(x) is concave if and only if
d2f(x)
≤ 0 for all x.
dx2
Separable Functions
• A function f(x1, x2, . . . , xn) is separable if
f(x1, x2, . . . , xn) = f1(x1) + f2(x2) + • • • + fn(xn)
• The separable function f(x1, x2, . . . , xn) is convex
(concave) if and only if each term is convex (concave).
 A linear function is both convex and concave.
 If the function f(x1, x2, . . . , xn) has some terms convex
and some terms concave, the function is neither convex or
concave.
Examples
a. f(x) = 4x2 – 20x + 10
b. f(x) = – 4x2 – 20x
c. f(x) = 8x3 +15x2 + 9x + 6
d. f(x) = 10x1 + 5x2 – 3x3 – 4x4
e. f(x) = 10(x1)2 + 5(x2)2 – 3x3 – 4x4
f. f(x) = 10(x1)2 – 5(x2)2 – 3x3 – 4x4
g. f(x) = – 10(x1)2 – 5(x2)2 – 3x3 – 4x4
Multiple Dimensions
To determine convexity in multiple dimensions we define the
following:
• The gradient of f(x), denoted by f(x), is the n-dimensional
column vector formed by the first partial derivatives of f(x)
with respect to each individual element of x.
f(x)
f(x) = 

 xj 
• The Hessian matrix, H(x), associated with f(x) is the n n
symmetric matrix of second partial derivatives of f(x) with
respect to the elements of x.
2f(x)
H(x) = 

x
x
 i j
Examples
h. f(x)= (x1)2 + (x2)2
i. f(x) = 2(x1)2 + (x2)2 + 3(x3)2 + x1x2 + 2x1x3 + 3x2x3
Quadratic Forms
1
f(x) = k + cTx + 2 xTQx
f(x) = 3x1x2 + (x1)2 + 3(x2)2
Q=H=




2 3 
3 6 
(When written in polynomial form the element qii is twice
the value of the coefficient of (xi)2, and the element qij and
qji are both equal to the coefficient of xixj.)
j. f(x) = 2(x1)2 + 4(x2)2
k. f(x) = – 2(x1)2 + 4x1x2 – 4(x2)2 + 4x1 + 4x2 + 10
l. f(x) = – 0.5(x1)2 – 2x1x2 – 2(x2)2
Determining Convexity or Concavity from Hessian
Convex
• f(x) is convex (strictly convex) if its associated Hessian
matrix is positive semi-definite (definite) for all x.
• H(x) is positive semi-definite if and only if
xTHx ≥ 0 for all x.
• H(x) is positive definite if and only if
xTHx > 0 for every x ≠ 0.
Concave
• f(x) is concave (strictly concave) if its associated Hessian
matrix is negative semi-definite (definite) for all x.
• H(x) is negative semi-definite if and only if
xTHx ≤ 0 for all x.
• H(x) is negative definite if and only if
xTHx < 0 for every x ≠ 0.
Neither Convex nor Concave
• f(x) is neither convex nor concave if its associated Hessian
matrix, H(x), is indefinite for some point x.
• H(x) is indefinite
if and only if xTHx > 0 for some x,
and (x– )THx– < 0 for some other x– .
Both Convex and Concave
• f(x) is both convex and concave only if it is linear.
Testing for Definiteness with Determinants
The ith leading principal determinant of H is the determinant of
the matrix formed by taking the intersection of the first i rows
and the first i columns. Let Hi be the value of this determinant.
Then
H1 = h11
H2 =




h11 h12 
h21 h22 
and so on until Hn is defined.
 H is positive definite if and only if:
Hi > 0, i = 1,... ,n.
 H is negative definite if and only if:
H1 < 0, H2 > 0, H3 < 0, H4 > 0, …
 H is indefinite if the leading principal determinants are all
nonzero and neither of the foregoing conditions are true.
Examples
f(x) = (x1)2 + (x2)2
2 0 
H(x) = 0 2 





f(x) = 3x1x2 + (x1)2 + 3(x2)2
H=




2 3 
3 6 
f(x) = 2(x1)2 + (x2)2 + 3(x3)2 + x1x2 + 2x1x3 + 3x2x3
4 1 2


H= 1 2 3
2 3 6
f(x) = – 2(x1)2 + 4x1x2 – 4(x2)2 + 4x1 + 4x2 + 10
f(x) = – 0.5(x1)2 – 2x1x2 – 2(x2)2
When does a Constraint Define a Convex Region?
• gi(x) ≤ bi defines a convex region if gi(x) is a convex
function.
• gi(x) ≥ bi defines a convex region if gi(x) is a concave
function.
• gi(x) = bi defines a convex region only if gi(x) is a linear
function.
When all the constraints individually define convex regions,
they collectively define a convex region.
When is a Local Optimum a Global Optimum?
minimize f(x)
maximize f(x)
subject to
subject to
gi(x) {≤, ≥, =} bi,
gi(x) {≤, ≥, =} bi,
i = 1, …,m
i = 1, …,m
 When f(x) is convex and
the set of constraints
defines a convex region.
 When f(x) is concave and
the set of constraints
defines a convex region.
• Otherwise, there is no guarantee that a specific local optimum
is a global optimum.
Problem Categories
1. Unconstrained Problems (classical optimization):
min{f(x) : x  Rn}
2. Nonnegative Variables:
min{f(x) : x ≥ 0}
3. Equality Constraints (classical optimization):
min{f(x) : hi(x) = 0, i = 1, . . . , p}
4. Inequality Constraints:
min{f(x) : hi(x) = 0, i = 1, . . . , p; gi(x) ≤ 0, i = 1, . . . , m}
Optimality Conditions
Unconstrained Problems: min{f(x) : x  Rn}
Necessary Conditions: Let f : n  1 be twice continuously
differentiable throughout a neighborhood of x*. If f has a
relative minimum at x*, then it necessarily follows that
(i) f(x*) = 0
(ii) The Hessian F(x*) is positive semidefinite.
Sufficient Condition: The Hessian F(x*) is positive definite.
Example
Find the relative extreme values of
3
2
f(x) = 3x1 + x2 – 9x1 + 4x2
2
Solution: f(x) = [9x1 – 9 2x2 + 4] = [0 0]
implies that x1 = 1 and x2 = – 2.
Checking x = (1, – 2), we have
F(1, – 2) =
18


 0
0
2 
2
2
 positive definite since vTF(1, – 2)v = 18v1 + v2 > 0
when v ≠ 0. Thus (1, – 2) gives a strict relative minimum.
Next consider x = (–1, –2) with Hessian
F(– 1, – 2) =
– 18 0




 0 2 
2
2
Now we have vTF(– 1, – 2)v = – 18v1 + v2 which may equal 0
when v ≠ 0. Thus the sufficient condition for (– 1, – 2) to be
either a relative minimum or maximum is not satisfied.
Optimality Conditions
Nonnegative Variables: min{f(x) : x ≥ 0}
Necessary Conditions (relative minimum of f(x) at x*):
f(x*)
= 0,
xj
if x*j > 0
f(x*)
≥ 0,
xj
if x*j = 0
Alternatively:
f(x*) ≥ 0, f(x*)x* = 0, x* ≥ 0
Example
2
2
2
Minimize f(x) = 3x1 + x2 + x3 – 2x1x2 – 2x1x3 – 2x1
subject to x1 ≥ 0, x2 ≥ 0, x3 ≥ 0
Solution: Necessary conditions for a relative minimum:
f
= 6x1 – 2x2 – 2x3 – 2
x1
(1)
0 ≤
(2)
0 = x1
(3)
0 ≤
(4)
0 = x2
(5)
0 ≤
(6)
f
0 = x3
= x3(2x3 – 2x1)
x3
(7)
x1 ≥ 0, x2 ≥ 0, x3 ≥ 0
f
= x1(6x1 – 2x2 – 2x3 – 2)
x1
f
= 2x2 – 2x1
x2
f
= x2(2x2 – 2x1)
x2
f
= 2x3 – 2x1
x3
From (4), either x2 = 0 or x1 = x2.
When x2 = 0, (3) and (7)  x1 = 0.
From (6), x3 = 0.
But this contradicts (1) so x2 ≠ 0 and x1 = x2.
Example (Continued)
Condition (6)  either x3 = 0 or x1 = x3.
If x3 = 0, then (4), (5) and (7)  x1 = x2 = x3 = 0.
But this situation has been ruled out.
Thus, x1 = x2 = x3 and from (2) either x1 = 0 or x1 = 1.
Since x1 ≠ 0, the only possible relative minimum of f occurs
when x1 = x2 = x3 = 1.
Must evaluate Hessian at x* = (1, 1, 1):
F(1, 1, 1) =
det(F) = 8, det




 6 –2 –2
 –2 2 0 
 –2 0 2 
 6 –2


–2 2








= 8, det(6) = 6
Determinants of all principal minors are positive definite so f is
strictly convex and has a relative minimum at x*.
Also, convexity implies that f(x*) = 1 is a global minimum.
Optimality Conditions
Equality Constraints: min{f(x) : hi(x) = 0, i = 1, . . . , p}
Jacobian matrix at x*:
h(x*)
=
x
h1(x*)
 x
 1•

•

•
hp(x*)
 x
 1
h1(x*)
• • •
xn 



hp(x*)
xn 

•
•
•
• • •
Lagrangian: L(x, )  f(x) + Th(x),
Necessary Conditions (relative minimum of f at x*):
If f and each component hi of h are twice continuously
differentiable throughout a neighborhood of x*, and if the
Jacobian matrix has full row rank p, then there exists a *  p
such that
L(x*, *)
= f(x*) + Th(x*) = 0
x
L(x*, *)
= h(x*) = 0

Example
min (max) f(x1, x2) = 2x1x2
2
2
subject to x1 + x2 = 1
2
2
Solution: Let L(x1, x2, ) = 2x1x2 + (x1 + x2 – 1).
L
= 2x2 + 2x1 = 0
x1
(1)
L
= 2x1 + 2x2 = 0
x2
(2)
L
2
2
= x1 + x2 – 1 = 0

(3)
Three equations in three unknowns:
Solving (1) for  we get  = – x2/x1 as long as x1 ≠ 0.
2
2
Substituting into (2) gives x1 = x2 .
2
From (3), we get 2x1 = 1.

x1 =  2/2, x2 =  2/2 and  = +1 or –1
depending on the values of x1 and x2.
Example (continued)
Four possible solutions for (x1, x2, ):
( 2/2,
2/2, – 1)
(– 2/2, – 2/2, – 1)
( 2/2, – 2/2, 1)
(– 2/2,
2/2, – 1)
By inspection:
(i) minimum value of f = –1 at both
x = (– 2/2, 2/2) and ( 2/2, – 2/2).
(ii) maximum value of f = 1 at both
x = ( 2/2, 2/2) and (– 2/2, – 2/2).
Second Order Optimality Conditions
Tangent Plane at x*: T = {y : h(x*)Ty = 0}
Second Order Necessary Conditions: Suppose that x* is a local
minimum of f(x) subject to h(x) = 0 and the Jacobian has rank p.
Then there exists a vector  p such that

f(x*) + Th(x*) = 0
and the Hessian of the Lagrangian
L(x*)  F(x*) + TH(x*)
is positive semidefinite on T; that is
yTL(x*)y ≥ 0 for all y  T
Second Order Sufficient Conditions (for unique local minimum):
L(x*) is positive definite on T.
Example (continued)
Hessian: L(x*) =
0


2
2 0
2


 + 
 (independent of x*)
0
0 2
Tangent Plane: T = {(y1, y2) : x*1 y1 + x*2y2 = 0}
At either minimizing point,  = 1 so L =
T = {(y1, y2) : y1 = y2},
yTLy
=
2


2
2
(y1, y2) 
2
2
,
2
2 y1
2
   = 8y1 ≥ 0
2 y2
Thus second order necessary and sufficient conditions for a
relative minimum are satisfied.
At either maximizing point,  = – 1, L =
–2


2
2
–2 ,
2
T = {(y1, y2) : y1 = –y2}, yTLy = –8y1 ≤ 0
L is negative definite on T  relative maximum.
Optimality Conditions
Inequality Constraints:
min{f(x) : hi(x) = 0, i = 1, . . . , p; gi(x) ≤ 0, i = 1, . . . , m}
Regular Point: Let x* be a point satisfying the constraints h(x*)
= 0, g(x*) ≤ 0 and let I be the set of indices i for which gi(x*) =
0. Then x* is said to be a regular point of these constraints if
the gradient vectors hi(x*) (1 ≤ i ≤ p), gi(x*) (i  I) are
linearly independent.
Necessary Optimality Conditions (Karush-Kuhn-Tucker
Conditions): Let x* be a relative minimum and suppose that x*
is a regular point for the constraints. Then there exists a vector
  p and a vector   m such that

f(x*) + Th(x*) + Tg(x*) = 0
Tg(x*) = 0
 ≥ 0
h(x*) = 0, g(x*) ≤ 0
Second Order Optimality Conditions
Necessary Conditions: Suppose that the functions f, h, g  C2
and that x* is a regular point of the constraints. If x* is a
relative minimum, then there exists a vector   p and a
vector   m,  ≥ 0 such that KKT Conditions hold and
L(x*)  F(x*) + TH(x*) + TG(x*)
is positive semidefinite on the tangent subspace of the active
constraints at x*.
Sufficient Conditions: Hessian matrix
L(x*) = F(x*) + TH(x*) + TG(x*)
is positive definite on the subspace:
T' = {{y : h(x*)y = 0, gi(x*)y = 0 for all i  I'}
where I' = {i : gi(x*) = 0, i > 0}.
Example
2
2
1 2
1
min f = 2x1 + x2 + 5 x3 + x1x3 – x1x2 + x1 – 2 x3
subject to
h1 = x1 + x2
= 3
2
2
g1 = x1 + x2
– x3 ≤ 0
g2 = x1 + x2 + 2x3 ≤ 16
g3 = x1
g4 =
≥ 0
x2
≥ 0
g5 =
x3 ≥ 0
Stationarity Conditions:
4x1 + x3 – x2 + 1
 2x2 – x1 
 2
1 
 5 x3 + x1 – 2 
–
1

3 0

0
–
+
1

1 1

0
0

4 1

0
–
+
2x1
 
1 2x2
 
 –1 
0

5 0

1
=
+
0
0

0
1

2 1

2
Complementarity Conditions:

2
2

1 x1 + x2 – x3 = 0
2 x1 + x2 + 2x3 – 16 = 0
3 x1 = 4 x2 = 5 x3 = 0
i ≥ 0, i = 1, . . . , 5
• The 5 complementarity equalities imply 25 = 32 combinations
may have to be examined (most of these are infeasible).
• Assume that only g2 is active so i = 0 for all i ≠ 2.
h1 = x1 + x2 = 3 means x3 =
13
2.
The third stationarity condition then yields
2 13
  
5  2 
1
+ x1 – 2 + 22 = 0  2 < 0 which is not feasible.
• Now consider the case where only g1 is active.
One possible solution is –x = (1, 2, 5).
5
The third stationarity condition gives 1 = 2, and either the
first or second stationarity conditions gives 1 = –13.
Thus –x with corresponding  and  satisfies the first order
necessary conditions for a relative minimum.
Check second order conditions –– examine Hessian:
 4 –1 1 
2 0 0 
 9 –1 1 
5






L(–x) =  –1 2 0  + 2 0 2 0 =  –1 7 0 
 1 0 2/5
0 0 0 
 1 0 2/5
Matrix L(–x) is positive definite on R3 so it must be positive
definite on the required tangent subspace.
Therefore, –x = (1, 2, 5) is a unique local minimum.
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