Reference

NBS

PUBLICATIONS

NBSIR 85-3206

A1110L. 034031

Global Solutions to Factorable

Nonlinear Optimization Problems

Using Separable Programming

Techniques

Garth P. McCormick

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Applied Mathematics

Gaithersburg, MD 20899

June 1985

Sponsored by

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards — QC— >r Applied Mathematics

, is Research Division

iUU

>urg, MD 20899

• U56

85-3206

1985

NBSIR 85-3206

' • • GLOBAL SOLUTIONS TO FACTORABLE

NONLINEAR OPTIMIZATION PROBLEMS

USING SEPARABLE PROGRAMMING

TECHNIQUES

Garth P. McCormick

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Applied Mathematics

Gaithersburg, MD 20899

June 1985

Sponsored by

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Applied Mathematics

Operations Research Division

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige. Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

HATIONAE BUREAU OF STANDARDS

LIBRARY '

/

I C{\

ABSTRACT

Many algorithms for obtaining global solutions to nonconvex optimization

problems have been proposed in recent years. The methods farthest along

computationally are those for separable problems. These use linear

programming codes to solve sequences of LP problems formed from piece-wise

linear approximations to the nonlinear functional forms. For a large class of

optimization problems, called factorable programming problems, it is possible

to create equivalent separable problems. This is done at a cost: additional

variables and constraints. In this paper the procedure for creating the

equivalent separable problems is outlined and a brief description is given of

a global solution algorithm due to Falk. A small example is given illustrating the above techniques. The example is also solved using a more direct

method. Application to the solution of nonlinear least squares is illustrated

with another example. Discussion of areas of research for improving the

efficiency of this approach concludes the paper.

i
TABLE OF CONTENTS

Page

ABSTRACT i

List of Figures iii

List of Tables iv

1. Global Solutions to Separable Problems 1

2. Factorable Programming Problems 5

3. Creating Equivalent Separable Programs 8

4. Obtaining Bounds on the New Variables 13

5. Computer Solution to Simple Problems 15

6. Direct Solution of Simple Problems 18

7. Application to Nonlinear Least Squares 26

8. Discussion of Results and Areas of Future Research 35

9. References 37

Appendix A Listings of Subroutine GETPHI and Data Input File for

Two Variable Problems

Appendix B Listings of Subroutines and Data File for Least Squares

Problems

ii
LIST OF FIGURES

Page

Figure 1 - Piecewise Linear Approximations 3

Figure 2 - Isovalue Contours of Two Variable Problem 7

Figure 3 - Convex Envelope of Sine Function in [0,10] 20

Figure 4 - A Convex Underestimating Function 22

Figure 5 - Isovalue Contours of Convex Underestimating Function 25

Figure 6 - Plotted Data for Example 29

iii
LIST OF TABLES

Page

Table I - Computer Effort Required to Solve LP Approximating

Problems . . 17

Table 2 - Data for Example 28

Table 3 - Data for Least Squares Problem in Separable Form 32

Table 4 - Data Relating to Solution of Least Squares Problem 34

iv
1 . Global Solutions to Separable Problems

Solving nonconvex programming problems, i.e., optimization problems where

local minimizers may not be global minimizers was thought not long ago to be

relegated to heuristic algorithms. (See [McCormick 1972a] for a survey of

some of these methods.) Recent investigators (e.g. [Falk and Soland 1969],

[Soland 1971], [Falk 1972], [Beale and Tomlin 1970], [Hoffman 1981], [Mancini

and McCormick 1976], [Mancini and McCormick 1979], [McCormick 1980]) have

developed rigorous algorithms for which convergence to global minimizers can

be proved. The theories are well-established and some computational results

are available.

The algorithms which have the most computational development are those which

solve separable optimization problems using linear programming codes.

Separable programming problems have the following form:

minimize F

Q (x) x e Rn

subject to Fi(x) < b^, i = 1, ..., m,

^j ** x

j ^ Lj, j - 1 , n, (Q)

where each F

i (x) is written

n

F

± (x) = l Fii (x i ), i = 0, 1, ..., m. j=l J

(A more general formulation allows for equality constraints: i.e. for

Fj_(x) = b^, i = m + 1, ..., p. However, for simplicity in presentation only,

the inequality constrained problem is considered in this section.)

1
The basic idea behind most of these methods is to approximate each nonlinear

functional by a piece-wise linear functional and solve the resulting problem

by solving a finite sequence of linear programming problems. The computer

programs used to solve the example of this paper implemented the algorithm of

Falk and is briefly described below. For more details the reader is referred

to [Falk 1972] or Appendix A in [Grotte 1978] .

The approximating problem of the original problem Q is obtained by replacing

each function F^j by a piece-wise linear approximation over the interval

[Jlj, Lj] . This involves selecting pj + 1 grid points yjQ > — , _,j p

> yTM in

3 [£j» Lj], where yj0 = and yj p = Lj. Suppose Xj* is some point in

[Aj, Lj]. Let yjk*, yj k*+1 , 0

ylc* be the unique values

where

x j* = 0 jk*yjk* + (1 " °jk* ) yjk*+l » 0 < 0jk* < 1 *

Then

F ij (x j*) " 9 jk*F ij^jk* ) + (1 G jk* ) F ij ( yjk*+1 )

Figure 1 represents this type of approximation.

There are obviously many ways these pj + 1 points can be selected. The

computer program implementing this algorithm allows the user to specify the

value

p

j . It then computes equally spaced points.

Let Kj = (0, 1, ..., Pj}

The approximating problem can be stated in terms of

the weights on the grid points, the ©jk.' s:

2
X

M

Figure 1. Piecewise Linear Approximations

3
n min

0

I l G jk Foj

j=l ks K

j

subject to

n

I l

j=l J keK^

J

I G jkF ij(yjk) * ^i> •••> m >

1 j — 1 »

• • • > n > (P)

>0, j = l, ...,n;kekj

and (for j = 1, . . .

,

n) at most two of the weights {Gj^, kekj) can be

nonzeros. If two are nonzero, they must correspond to adjacent grid points.

This last constraint is the adjacency weighted restriction (AWR).

If it were not for the AWR, the approximating problem (P) would be a simple

linear programming problem with variables { 0 * * This restriction is

necessary in order that the piecewise linear approximation indicated be

valid

.

In [Falk 1972] a branch and bound algorithm for solving (P) is presented. It

involves solving a sequence of linear programming problems without the

adjacency weight restrictions. The details of his algorithm however, will not

be pursued here.

Most nonlinear programming problems are, in their simplest formulation, not

separable. For a large class of problems, called factorable programming

problems it is possible to create separable programs which are "equivalent" to

the original ones in the sense that local minimizers are in a one-to-one

correspondence. This is done at a cost; an increase in the number of

variables and constraints.

The description of factorable optimization problems is in the next section.

4
2 . Factorable Programming Problems

A factorable programming problem is one which can be written in the following

form:

N-l N-l p

min fN

(x) = l TN>p [fp(x)] + £ J %, p,q[f q(x ) ] * VN, q , p

[ f p( x > 1 p=l p=l q=l

subject to

a^ < fj(x) = xj < bj, j = 1, n, (2.1)

j“l j“l P a

j < f j( x ) E l T j,pt f p( x ^ + I l Uj,P,q, [ f qCx >] vj,q,p[ f pCx )J < bj,

p=l P=1 q=l

j = m+1, N-l where some of the aj's are possibly equal to -°°, and the

T's, U's and V’s are scalar functions of one variable.

Most practical nonlinear programming problems are factorable. The reader is

referred to [McCormick 1975J and [Ghaemi and McCormick 1979] for a fuller

discussion of this subject.

Consider the following nonlinear programming problem:

min ERF (x^+x 2 ) + sin (x^) • exp(-.5x2)

( x

l> x 2 )

subject to

-(xi+x2)2 < -10 (2.2)

0 < x^ < 10, 0 < X2 < 10,

where

k = 2//ir

,

z ERF(z) = k/ exp(-t2)dt

o

5
Written as a factorable programming problem f = f -( x )) this has the form:

J J

min f 5 = ERF(f 3 ) + sin (f]_) • exp(-. 5 f 2 ) (xi,X2 ) (2.3)

subject to

0 < f]_ = xi < 10

0 < f

2

= X 2 < 10

—00 < f

2

5 f]_ + f 2 < + 00

-« < f 4

= — (

f

3 ) 2 < -10

The isovalue contours of the objective function are plotted in Figure 2.

There are two local minimizers, one at (10,0) with objective function value

.45599, and another at approximately (

3

tt

/

2 , 0) with objective function value

approximately 0. If the inequality constraint (x^ + X 2 )^ > 10 were removed

from the problem, (0,0) would also be a local minimizer. The point (3tt/ 2,0)

is the global minimizer in both cases with a function value slightly less than

zero

.

6
Figure 2. Isovalue Contours of Two Variable

Problem

7
3 . Creating Equivalent Separable Problems

The techniques which can be used to convert most nonlinear programming

problems into "equivalent" separable programs are part of the tradition of

optimization. The origins of these simple techniques are lost in the past. A

description of two of these are contained in [McCormick 1972b]. Below is a

summary statement of these techniques.

The method for creating equivalent separable problems has two basic steps

which are used repeatedly until they can no longer be applied.

Step 1 . If the optimization poblem has a product term of the form

q^(x) • q2 (x), replace that product by (zi) 2-(z2 )^ wherever it occurs,

and introduce the equality constraints

q i ( x ) = zi + Z2

q 2 (x) = z 1 - z2

.

The problem variables are augmented to include z\ and Z 2

-

Step 2 . If the optimization problem has a term of the form T[t(x)] where T(t)

is a scalar function and t(x) is a scalar function of more than one

variable, replace T [ t ( X) ] by T(y) wherever it occurs, add the

equality constraint t(x) = y, and augment the variable list with y.

Consider the example given earlier. Applying step 2 for the expression

t(x) = x^-hc 2 yields the program

8
(3.1)

min ERF(yl) + sin (xi) • exp (-.5x2)

( x l,X2,yi)

subject to

-(yi) 2 < -io

x i + x 2 = yi,

0 < x-^ < 10,

0 < x 2

< 10.

Application of Step 1 to the product term in the objective function yields the

equivalent separable program

min ERF (y^) + (z^)2 - (Z 2) 2

( x l> x2»yi) z l> z 2) (3.2)

subject to

-(yi) 2 <-io

x i + x 2 = yi,

sin (x

x ) = zi + z 2 ,

exp(-.5x2 ) = zi - z 2 ,

0<x 1 <10, 0<x 2

< 10

.

For this simple problem, separation occurred after just two applications of

the separating techniques. Often the situation is more complicated.

The "equivalence" of the problems is shown in [McCormick. 1972c] , where it is

shown that local minimizers are in one-to-one correspondence. For the purpose

here it is sufficient to emphasize that if (x^ ,X 2 ,yi , , Z 2 ) is a local

(global) minimizer for (3.2), then (x^,x2) is a local (global) minimizer for

(3.1).

9
Not all nonlinear programming problems can be separated by repeated

application of steps 1 and 2. In particular, if a program cannot be written

as a factorable optimization problem it probably cannot be separated.

Certainly, if it can be written in factorable form, separation is possible.

Make the correspondence yj = Xj, j = 1, ..., m. Then using steps 1 and 2 on

the factorable programming problem (2.1) yields directly

N" 1 N-l p N, 1 9 N,2 0 min l TN >p (yn ) + l l Uz q,p)

2 ~ (Zq,p) 2

] (3.3)

(y,z) p=l p=l q=l

subject to the inequality constraints

a

j < yj < bj, j = 1, ..., N-l

and the equality constraints

yj

=

l T

j, P (yp

) + I ? ^ Z q,p^

2

p=l p=l q=l

j = n+1, . .. , N; (3.6)

p

= 1 , . .

.

,

j—1

,

q = 1 ,

* • • , p.

rp Here y = (x^, ..., xm , ym4.^, •••, y^-i) , and z is the vector containing

j > 1 the Z n n Z _ ’s appearing above.

Application of these equations to the example problem given in form (2.3)

yields the separable problem

U j,p,q (y

q

) j,l 3,2

z + z q.p q,p

v j»p,q (y

p

) J . 1

7

j » 2 z q»p

z q,p

(3.4)

- (Zq’p) 2

], j=m+l , ..., N-l; (3.5)

10
min ERF(y (3.8) 3 ) + (Zi >2 ) 2 “ ( z i } 2)

2

subject to

0 < y^ < 10

0 < y2

< 10

-00 < y3 < +»

-°»<y^<-l-<» (3.9)

y3 = yi + y2>

Y4 = (y3 ) 2 (3.10)

5,1 5,2

sin(yi) = Z 1>2 + z l,2

5,1 5,2

exp(-.5y2 ) = Z 1>2 “ z l,2 ,

where the problem variables are

5,1 5,2

(yi» y2> y3» yu> z i,2» z i,2 )

Problem (3.8) is similar to the separable problem (3.2) obtained directly by

applying steps 1 and 2 except that it has one variable more and one more

equality constraint. This is an example of a general modification which

should be made to the separation rules based to create (3.3). If some fj

(j = m+1, ..., N-l) does not enter into the computation of any future f- the

introduction of the new variable yj should be omitted, and no equality

constraint of the form (3.5) should be included. The simple bound in (3.4) in

this instance should remain

a-j < 3-1

Ip=l

T j,p(yp)

3 -1 P j,l

+ I I KZ q , P ) 2 ' p=l q=l

j , 2 „ (Z

q ,p)

2

) < b.

11
Using this modification, problem (3.8) above would be changed in that would

no longer be a variable, (3.10) would be omitted, and (3.9) would become

-°° < ~(y3)2 < -10. It is then equivalent, except for the variable names, to

problem (3.2).

12
4. Obtaining Bounds on the New Variables

Algorithms which obtain global solutions to separable optimization problems

invariably require upper and lower bounds on the variables. (An inductive

process to do this when all original variables {xj} are bounded is given

below)

.

The basic assumption here is that it is possible to compute the suprema and

infima of T's, U's, and V's used in defining the factorable programming

problem (2.1).

Define

£(T,p,j) = inf Tj jp (y) s.t. a

p

< y < bp,

u(T,p,j) = sup T j}p (y) s.t. a

p

< y < b

p

,

£ (U, p, q, j ) = inf Uj }P}q (y) s.t. a

q < y < bq , u(U,p,q,j) = sup Uj jPjq (y) s.t. a

q

< y < b

q

.

Analagous notation is used for the other quantities.

Then the lower bound on y^ is given by

yj > max (aj,cj)

where

j-1 j-1 p

c

j = l A(T,p,j) +1 l max [Z(U,p,q, j)«Z(V,p,q, j),i(U,p,q, j)«u(V,p,q, j), p=l p-1 q=l

u(U,p,q,j)«A(V,p,q,j), u(U,p,q,j)»u(V,p,q,j)]

Analogous bounds are computed for the other quantities.

13
Consider the separable problem created in section 3. Since y3 ^1

the bounds on both and y2 are 0 and 10,

0 < y3 < 20.

(Recall y^ was eliminated from the problem as unnecessary). Now

5,1

Zi, 2 = [sin(y

1

) + exp(-.5y2 ) ] /2

and

Zl’,2 = [sinCyj^) - exp(-.5y2 )]/2.

Since

-1 = inf sin(y^) < sinCy^) < sup sin(y]_) = 1,

where

0 < y^ < 10

and

0 = inf exp(-.5y2 ) < exp(-.5y2 ) < sup exp(-.5y2

) < 1 where 0 < y

it follows that the bounds on these new variables are

5.1 , -.5 < Zi j2 < 1»

5.2

-1. < Z 1>2 < - 5 *

+ y2 and

< 10 ,

14
5 . Computer Solution To Simple Problem

The computer program which implements Falk's branch and bound procedure (see

section 1) is called NUGLOBAL and is available from the Operations Research

Division of the National Bureau of Standards. It uses a linear programming

package called SEXOP written by Dr. Roy E. Marsten.

The user is required to write a FORTRAN subroutine (GETPHI) which supplies the

values of the nonlinear functions defining the separable optimization problem.

Input to this routine is the constraint index i, variable index j, and

variable value Xj. The output is Fij(xj). For the simple two variable

example the SUBROUTINE GETPHI is listed in Appendix A.

The user is also required to supply a data file. The one used for this

example is given in Appendix A. (This corresponds to the Run 3 to be

discussed next.)

Computer instructions on how to use the NUGLOBAL system are in [Hoffman

1975] .

For notational convenience and to conform to the separable format, problem

(3.2) is rewritten

min ERF(x3 ) + (X4) 2 - (X5) 2

( x l> X5)

15
Subject to:

X 1 + x 2 “ x 3 = 0

-sinCx-^) + X4 + X5 = 0

-exp(-.5x2 ) + x^ - X 5 = 0

- x 3 <-10

0 < xL

< 10 ,

0 < x2

< 10,

0 < x3

< 20 ,

-.5 < x^ < 1 ,

1 < x5

< .5.

For "Run 1" the first variable was "divided into nine variables", i.e.

Pl=8 (see Section 10). That is, in the linear programming problem

01 , 0 , •••, ©1,8 "represented" *- 1

. Variables x2 -x 3 .x 4.x 5 were replaced by 6

variables each. The resulting linear programming problems had 33 0

variables

.

To get the global minimizer to problem (P) required the solution of fifteen

linear programming problems. The solution obtained was

x* = (5.0, 0.0, 5.0, 0.020538, -0.97946).

The value of the piece-wise linear approximation to the objective function was

(.052861). The value of the true objective function at this point is

(0.04107572)

.

The second run ("Run 2") used pj = 10, j=l, •••, 5. This created linear

programming problems with 55 0 variables. This required the solution of 12

linear programming problems to obtain the global solutions to Problem (P)

.

16
The solution obtained was the same as run 1, x*=(5.0, 0.0, 5.0, 0.020538,

-0.97946). The value of the piece-wise linear objective function was

(0.04402). The true objective function value there is, as before, (0.04107).

These two values are closer than in run 1 because of the finer grid

approximation in Run 2.

In Run 3, pj = 25, j = 1, ..., 5. Each Xj was represented by 26 0 variables.

Each linear programming problem had 130 variables. Nine linear programs were

required to find the solution x* = (4.8, 0.0, 4.8, 0.0019177, -0.99808). The

true objective function value there is (0.00383) with piece-wise linear

approximate value of (0.00455). These results are summarized in Table 1.

# of LP

Vars.

LPS

Solved

X

1 x2 x

3 x

4 x5

Run 1 33 15 5.0 0.0 5.0 .0205 -0.9795

Run 2 55 12 5.0 0.0 5.0 1 0205 -0.9795

Run 3 130 9 4.8 0.0 4.8 .0019 -0.9981

Theoretical

Solution

4.71

=3tt / 2

0.0 4.71 0.0 -1.0

Table 1 - Computer Effort Required to Solve LP Approximating Problems

17
6 . Direct Solution of Simple Problem

Another approach for finding global minimizers to nonconvex programming

problems (see [McCormick 1976] for more details) is to solve a sequence of

"convex underestimating problems." Convex programming problems are problems

which can be written:

where the functions

are convex functions.

minimize f(x)

subject to

gj_(x) >0, i = 1, . •

•

,

m

m

f, {-gi>i=l

The nice property of convex programming problems is that local minimizers are

global minimizers. The isovalue contours resemble ellipses, parabolas, or

lines. It is clear from Figure 2 that the objective function of problem (2.2)

is not a convex function.

The constraints of a convex programming problem define a convex set for the

feasible region. For this simple problem, the constraint (x^ + X 2 )^ > 10 does

not define a convex set. However, the intersection of the set of points

satisfying this with the feasible region is a convex set.

A convex underestimating problem for a general problem is one where the

objective function is convex and underestimates the given objective function

in the feasible region and where the constraint region is a convex set and

18
contains the original feasible region. Presented in [McCormick 1976] is an

algorithm for finding a convex underestimating problem when the original

optimization problem is factorable with bounded variables.

The key to implementing this approach is the ability to compute, for a

function of a single variable in an interval, its convex envelope. This is

the highest convex function which underestimates the function in the interval.

Thus, for example, the convex envelope of sin(x^) where 0< x^ < 10 is

This is shown in Figure 3, where the notation vex f(») is used. Proper

notation would indicate the interval. Thus vexsin [xj_, 0, 10] should be used,

but for notational simplicity this is not done.

-0.21723 xi, 0 < xi < 4.49342

vexsin(x

1 ) = sin^), 4.49342 < xi < 4.79946

(-1.41353 +0 .08695x l ) , 4.79945 < xi < 10

-.21723x 0 < x < 4.493719

Sin(x) 4.493419 < x < 4.7994496

-1.4135281 4.7994496 < x < 10.

+ .0869507x

19
-0.217x

0

<

x<

4.493

Sin(x)

4.493<

x<4.799

Figure 3. Convex Envelope of Sine Function

[ 0 , 10 ]

20

Vexsin

(x)
Using the general techniques in the above referenced article, a convex

underestimating function for the objective function of example problem (2.2)

will be constructed.

Since 0 < x^ < 10 and 0 < x2 < 10, then 0 < x^ + x2 < 20. Consider

X!+x2

<K X 1 + x2 ) = k- / exp(-t 2 )dt

o

in this interval. It is clear from Figure 4, that .05 (x^ + x 2 ) is a "high

convex underestimating function in this interval to a close

approximation.

21
Figure 4. A Convex Underestimating Function

22

(Zx

+

lx)
Since 0 < xl < 10 , then -1 < sin (xi) < 1 . Since 0 < X2 < 10 , then

Thus

,

or

c x = exp(-5) < (-.5x2) < 1.

[sin(xi) + 1] [exp(-.5x2) —c i ] < 0,

where

sin(xj^) • exp(-.5x2) > -exp(-.5x2) + cx sin(x^) + cx

> “1 + c

2x 2 + C 1 [ vexsin (x^)] + C]_ (6.2)

c

2 = (-c x + 1)/10 = .0993262.

Note that -1 + .099326x2 is the convex envelope of -exp(-.5x2) in the

interval [0, 10]

.

Also

,

[sin(c 1 ) - 1] [exp (-.5x2) - 1] > 0,

or

sin(x^) • exp(-.5x2) > exp(-.5x2) + sin(xx) - 1

> exp(-.5x2) + vexsin (xx) -1. (6.3)

Since the maximum of two convex functions is convex, a convex underestimating

function for the product takes the form (combining (6.2) and (6.3))

sin(xx)» exp(-.5x2) > max[cx- l + C 2X 2 + cxvexsin(xx) , exp(-5x2) + vexsin(xi)-l]

.

(6.4)

Using this with (6.1) gives the full convex underestimating function:

Xl4x2

k J exp(-t^)dt + sin(x^) • exp(-.5x2) (6.5)

o

> .05(x^ + X 2 ) + max[cx~l + C 2X 2 + cxvexsin(xx) , exp(-.5x2) + vexsin(xx)~l J

•

Isovalue contours of this function are plotted in Figure 5.

23
The global minimizer for function (6.5) in the rectangle 0 < xl * 10

»

0 < x2

< 10 is (4.6623681, 0.). The first component is a solution of the

equation arccos (x^) = -.05. This is to be compared with the true solution of

the original problem (4.712389, 0.).

Development of the convex underestimating function above took no advantage of

the restriction that

-(x x + x

2

) 2

< -10

must hold. A convex underestimating function of -(x^ + x 2 ) 2 in the rectangle

0 < x±

< 10, i = 1, 2

is

-20 (x^ + x2

) < ~( x i + x 2 )^.

Thus the constraint can be imposed

-20 (x L + x

2

) < -10,

or x l + X

2

> *5.

The convex underestimating function of the objective function of (2.2) can

now be restricted to the region where .5 < (x^ + x 2 ) < 20. This gives the

tighter convex underestimating function

.024593(x 1 + x 2 ) + .5091315.

Using this with (6.4) gives a tighter convex underestimating function and the

resulting solution of this problem is (4.687793, 0.). The first component is

a solution of arccos (x^) = -.024593.

24
CD

CD

CD

O

II

co .E

<9 E

o

ii CD —

O X

oo

CD

Csl

Figure 5. Isovalue Contours of Convex

Underestimating Function

25

Global min
7 . Application To Nonlinear Least Squares

An important source of nonlinear programming problems is the area of parameter

estimation. In particular the finding of the "best” parameter values which

define a functional form often gives rise to an unconstrained, nonconvex

optimization problem.

\

Let x be a vector of "independent” variables, and y a "dependent” variable

which is thought to be related to x via the equation

y

= f (x,E),

where E is a vector of parameters and f is a predetermined functional form.

Let {y^}, i = 1, ...» r be observations of y simultaneous with observations

{x-^} , i = 1, . .

. , r of the independent variables. There is a body of theory

(See [Goldfeld and Quandt 1972J) which states, under the appropriate

assumptions, that the "best" values of (E j } are those which solve the problem

r minimize J [y t - f( X;L ,E)] 2

. E i=l

In the reference above is also a theory of the probability distribution of the

point estimate given by the above problem.

When f is linear in E, this is the well-known linear least squares problem and

the function minimized is a positive (semi) definite quadratic form. Local

minimizers of this are global minimizers.

26
When f is nonlinear in the parameters {Ej} t ^ie problem is much more

complicated and the possibility that local, but not truly global solutions of

that problem exist must be considered. When the Ej's are physical constants

to be estimated, as in many scientific applications, this difficulty has

serious implications.

In this section a small parameter estimation problem is considered. The

general separation method given in Section 3 plus some ati hoc techniques are

used to create an equivalent separable problem. This is in turn solved by the

solution algorithm of Section 1.

It should be noted that the number of variables in the separated problem is

greater than or equal to the number of parameters to be estimated plus the

number of observations (r). If the function f is complicated, more separation

is required and this can give rise to a separable problem in a large number of

variables with an accompanying large linear approximation problem.

In Figure 6 is plotted the fraction of college men married versus age. The

data for the problem is in Table 2. It is thought that the dependent variable

y (fraction of college men married) is related to the independent variable x

(age in years) by the functional equation

y = p$[(x-y)/o]

where

y

<Kz) = J

<J>(t)dt, —oo

and

4>(t) = (2ir) - * 5 exp (-t 2 /2).

(The normal density function.)

27
The parameters, whose best values are to be estimated from the data of Table 2 are p (the ultimate fraction of these married), y (the mean of the

distribution) and a (the standard deviation).

i i 2 3 4 5 6 7

x

i 14.6 16.8 18.7 20.6 23.1 27.1 32.0

y± 0.000 0.004 0.015 0.075 0.315 0.571 0.737

Table 2 Data for Example

28
Q3iaavi/\l N3IAI 3931103 NOIlOVad

Figure 6 Plotted Data for Example
The optimization problem to be solved is

7 min l {yi ”p$([xi - y]/a)} 2

. (7.1)

(p,6,p) i=l

As a first step in converting this to a separable problem set

v± = p$ [(x

± - y)/a], i=l , ...» 7, (7.2)

and

z ± = (x i - y)/o , i=l , ..., 7. (7.3)

A trick not mentioned in section 3 but one which is often useful is to

transform a product into a sum by taking logarithms. Doing this to (7.2)

yields

By setting

In v^ = In p + In $(zi), i=l, •••, 7.

y = w ]+w 2 >

1 /o = OJ

]_ “ m 2 ,

the original nonseparable problem becomes

7 minimize £ (y^ - v^) 2

i=l

subject to

ln p - In v

i + In <I>(z i )=0, i=l , ..., 7

x i (a“^)-zi ~(o)^)^ + ( 012 )^ = 0, i=l, ..., 7

y -a) a) 2 ~ 0

]_+o3 2 = ®

where the minimization is performed over (y , 1/a, ln p, {v^} , {z^}, u)^, 0) 2 )*

30
Note that the separated problem contains 1/a and In p instead of a and p.

This was done because the new formulation is equivalent and is linear in the

transformed parameters (which are variables of the optimization problem).

From the physical model it is obvious that the parameters are constrained as

p>0, a> 0, CKp <1. If the data is consistent with the model it is expected

that these constraints are implicit and will not be binding at the solution.

One requirement of the global minimization technique outlined in Section 1 is

that lower and upper bounds are required on the problem variables. It should

be pointed out that this can be an advantage since often, from knowledge of

the problem, good bounds are available. This can reduce the region in which

optimization takes place and thus cut down on the amount of computer effort

required to solve it.

From the data, obvious bounds on the original three parameters are:

18<|j<30, l<a<10, and .5<p<l. The techniques of Section 4 are then used to

place bounds on the new variables of the separated problem. The names of the

variables of the separated problem, their xj indices and their upper and lower

bounds are given in Table 3.

31
ri,

nd

j

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

Variable

Name

Lower

Bound

Upper

Bound

Cuts

Run 1 CutsRun 2

y 18. 30. 1 1

1/a .1 1 . 1 1

In p -.693 0 . 1 1

V1 .0006 .37 5 5

v2 .0006 .45 5 5

v3 .0006 .76 5 5

v4 .0006 1 . 5 5

v5 .0006 1 . 5 5

v

6 .0009 1 . 5 5

v7 .280 1 . 5 5

Z

1 (-3.) -.34 3 5

z 2 -3. -.12 3 5

z 3 -3. 0.7 3 5

z 4 -3. 2.6 3 5

z 5 -3. 3. 3 5

z 6 -2.9 3. 3 5

z

7 .2 3. 3 5

m 8.0 16. 5 5

0) 8.0 16. 5 5

Table 3: Data for Least Squares Problem in Separable Form

32
A modified Newton method was applied directly to problem (7.1) and gave the

solution

y = 24.4, a = 3.28, p = 0.73.

Two computer runs were made on the separated problem. The data files and

subroutine are in Appendix B.

The number of cuts for each of the nineteen variables are given in Table 3.

The number of 0-variables in the linear programming problems associated with

each of the variables in the separated problem is equal to the number of cuts

plus one. Thus in Run 1 each linear programming problem had 88 variables, and

in Run 2 each had 102 variables. The lower bound for in Run 1 was -3 and

it was -5 in Run 2.

For Run 1 the approximate solution was obtained after the solution of 41

linear programming poblems and was

y = 22.19, a = 4.28, and p = .57.

For Run 2 the approximate solution was obtained after the solution of 62

linear programming problems with values

y = 22.54, a = 3.31, and p = .72.

These results are summarized in Table 4.

33
Method U a P

# of

Variables

MP Problems

Solved

Newton 24.14 3.28 0.73 3 l(NLP)

Run 1 22.19 4.28 0.57 88 4 1 ( LP

)

Run 2 22.54 3.31 0.72 102 62 (LP)

Table 4: Data Relating to Solution of

of Least Squares Problem

34
8. Discussion of Results and Areas of Future Research

In this paper it was shown how general nonlinear programming problems can

automatically be converted to equivalent separable problems when they consist

of factorable functions. Global solutions to linear approximations of these

separable programs are then obtained using a branch and bound technique with

linear programming subproblems. Two small examples were solved this way

illustrating the feasibility of the approach.

First, experiments should be made on the placing of the points which determine

the piece-wise linear approximations to the nonlinear functions of a single

variable. In solving example (3.1), refinement of the grid from Run 1 to Run

2 did not result in any improvement in the solution approximation (see Table

1). The final answer obtained (Run 3) only agreed with the correct solution

to one significant place even though the number of 0 variables approximating

the nonlinear functions was increased significantly. The reason for this was

that the grid refinement strategy used allowed only for equally spaced

intervals. Judicious placement should have as a result better accuracy and

fewer O-variables.

Recent efforts in this area are encouraging. In [Grotzinger 1981] problem

(1.2) was solved to the accuracy (4.20,0.) requiring 11 linear programming

problems none of which had more than 25 O-variables.

35
The second area of improvement is to lower the effort required to solve the

linear programming problems by using the inverse matrix (basis) information

available from similar LP's solved by the branch and bound procedure. This

should considerably reduce the computation time. No advantage of available

partial basis information was used in solving the problems.

Advantage can also be taken of this information when new variables are added

to improve the linear approximation.

Until efficient nonlinear programming codes are available, using linear

programming codes as subroutines to solve general nonlinear programming

problems seems a reasonable approach and one which could be implemented on a

production basis without much additional effort.

36
9. References

1. Beale, E.M.L., and John Tomlin, Special Facilities in a General

Mathematical Programming System for Nonconvex Problems Using Ordered Sets

of Variables in (J. Lawrence ed.) Proc. of the Fifth International

Conference on Operations Research (1970) pp. 447-454, Tavistock

Publication, London.

2. Falk, James E. and Richard M. Soland, An Algorithm for Separable Nonconvex

Programming Problems, Man. Sc. 15 (1969) pp. 550-569.

3. Falk, J.E., An Algorithm for Locating Approximate Global Solutions of

Nonconvex, Separable Problems, Technical Paper Serial T-262, Institute for

Management Science and Engineering, the George Washington University,

Washington, DC, April 1972.

4. Ghaemi, Abofazl, and Garth P. McCormick, Symbolic factorable SUMT: What

is it? How is it used?. Technical Paper Serial T-402, Institute for

Management Science and Engineering, The George Washington University,

Washington, DC, May 1979.

5. Goldfeld, Stephen M., and Richard E. Quandt, Nonlinear Methods in

Econometrics North-Holland, Amsterdam, 1972.

6. Grotte, Jeffrey H., A Computer Program for Solving Separable Nonconvex

Optimization Problems, IDA Paper P-1318, Institute for Defense Analysis,

Arlington, Virginia, January 1978.

7. Grotzinger, Stephen J., Personal Communication.

8. Hoffman, Karla L., NUGLOBAL—USER’S GUIDE, Technical Memorandum Serial

TM-64866, Institute for Management Science and Engineering, The George

Washington University, March 1975.

37
9.

Hoffman, Karla L., A Method for Globally Minimizing Concave Functions

Over Convex Sets, Math. Prog . 20 (1981), pp. 22-32.

10. Mancini, Louis J. and Garth P. McCormick, Bounding Global Minima, Math .

Oper. Res . 1 (1976) pp. 50-53.

11. Mancini, Louis J. and Garth P. McCormick, Bounding Global Minima with

Interval Arithmetic, Operations Research 27 (1979) pp. 743-754.

12. McCormick, Garth P., Attempts to Calculate Global Solutions of Problems

that May Have Local Minima, in (F.A. Lootsma, ed.) Numerical Methods for

Nonlinear Optimization , 1972, Academic Press, New York, pp. 209-221.

13. McCormick, Garth P., Converting General Nonlinear Programming Problems to

Separable Nonlinear Programming Problems, Technical Paper Serial T-267,

Institute for Management Science and Engineering, The George Washington

University, Washington, DC, June 1972.

14. McCormick, Garth P., Optimal Design of a Corrugated Transverse Bulkhead;

An Example of the Use of the Factorable Programming Language, Technical

Paper Serial T-313, Institute for Managements Science and Engineering,

The George Wahington University, Washington, DC, 1975.

15. McCormick, Garth P., Computability of Global Solutions to Factorable

Nonconvex Programs: Part I —Convex Underestimating Problems, Math .

Prog . 10 (1976) pp. 147-175.

16. McCormick, Garth P., Locating An Isolated Global Minimizer of a

Constrained Nonconvex Program, Math , of Oper . Res., 5(1980)

pp . 435-443.

17. Soland, Richard M., An Algorithm for Separable Nonconvex Programming

Problems II: Nonconvex Constraints, Man. Sc. 17 (1971) pp. 759-773.

38
S-114A (REV. 2 -80 ) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) NBSIR 85-3206 JULY 1985

4. TITLE AND SUBTITLE

Global Solutions to Factorable Nonlinear Optimization Problems Using Separable

Programming Techniques

5. AUTHOR(S)

Garth P. McCormick

6. PERFORMING ORGANIZATION (If joint or other than N BS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. ContractSSSXSSK

N00014-7 5-C-0611

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State. ZIP)

National. Bureau of Standards

Center for Applied Mathematics

Operations Research Division

Gaithersburg, MD 20899

10. SUPPLEMENTARY NOTES

~1 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Many algorithms for obtaining global solutions to nonconvex optimization problems

have been proposed in recent years. The methods farthest along computationally are those for separable problems. These use linear programming codes to solve sequences

of LP problems formed from piece-wise linear approximations to the nonlinear

functional forms. For a large class of optimization problems, called factorable

programming problems, it is possible to create equivalent separable problems. This

is done at a cost: additonal variables and constraints. In this paper the procedure

for creating the equivalent separable problems is outlined and a brief description is

given of a global solution algorithm due to Falk. A small example is given

illustrating the above techniques. The example is also solved using a more direct

method. Application to the solution of nonlinear least squares is illustrated with

another example. Discussion of areas of research for improving the efficiency of

this approach concludes the paper.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon . branch and bound; factorable functions; mathematical programming; nonconvex

programming; nonlinear least squares; nonlinear optimization; separable optimization.

13. AVAILABILITY

| XI Unlimited

I | For Official Distribution. Do Not Release to NTIS

I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

[~X] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

45

15. Pnc«

$8.50

UlCOUM.OC