ON PORTFOLIO OPTIMIZATION USING FUZZY DECISIONS
*)Supian SUDRADJAT and Vasile PREDA
Departement of Mathematics, Faculty of Mathematics and Natural Sciences Padjadjaran University, Bandung, Indonesia Jl. Raya Bandung Sumedang km.21 Bandung 40600 Indonesia, email :
adjat03@yahoo.com
Faculty of Mathematics and Computer Science Bucharest University Romania 14 Academiei Str., Sector 1, Bucharest 010014 email : preda@fmi.unibuc.ro
Abstract.
We consider stochastic optimization problems involving stochastic dominance constraints. We develop portfolio optimization model involving stochastic dominance constrains using fuzzy decisions and we concentrate on fuzzy linear programming problems with only fuzzy technological coefficients and application/implementation of modified subgradient method to fuzy linear programming problems.
AMS subject classifications.
Primary, 90C15, 90C29, 90C46, 90C48, 90C70;
Secondary, 46N10, 60E15, 91B06
1. Introduction
The problem of optimizing a portfolio of finitely many assets is a classical problem in theoretical and computational finance. Since the seminal work of Markowitz [11, 2] it is generally agreed that portfolio performance should be measured in two distinct dimensions: the mean describing the expected return, and the risk which measures the uncertainty of the return. As one of theoretical approach to the portfolio selection problem is that of stochastic dominance (see [18, 7]).
The rest of the paper is organized in the following manner. Section 1 provides some newly obtained results on stochastic dominance, motivation for posing the portfolio selection problem in the fuzzy decisions theory framework. Section 2 describes the formulation of the portfolio selection problems. Section 3, consider an overview of portfolio problem fuzzy technological coefficient. Section 4, give a solution of defuzzificated problems.
2. Portfolio problem
Let
R
1,...,
R
n be random returns of assets1
,...,
n
. We assume that
E
[
R
j
]
<
¥
for allj
,...,
=
1
n
. Our aim is to invest our capital in these assets in order to obtain some desirable characteristics of the total return on the investment. Denoting byx
,...,
1x
n the fractions of the initial capital invested in assets1
,...,
n
we can easily derive the formula for the total return:
(2.1)
R
(
x
)
=
R
1x
1+
...
+
R
nx
nThe set of possible asset allocations can be defined as follows:
X
{
x
n:
x
...
x
n1
,
x
j0
,
j
1
,...,
n
}
1
+
+
=
³
=
Î
=
R
.The mean return
[
(
)
]
)
(
x
E
=
R
x
m
,
then the resulting optimization problem has a trivial and meaningless solution: invest everything in assets that have the maximum expected return. For these reasons the practice of portfolio optimization resorts usually to two approaches.
[
(
)
]
)
(
x
V
=
ar
R
x
r
,
but many other measures are possible here as well.
The mean–risk portfolio optimization problem is formulated as follows:
(2.2)
max
(
x
)
(
x
)
X
xÎ
m
-
lr
,Subject to
x
Î
X
.Here,
l
is a nonnegative parameter representing our desirable exchange rate of mean for risk. If
0
=
l
, the risk has no value and the problem reduces to the problem of maximizing the mean. If
0
>
l
we look for a compromise between the mean and the risk.
The second approach is to select a certain utility function
u
:
R
®
R
and to formulate the following optimization problem(2.3)
max
[
u
(
R
(
x
)
)
]
X
x Î
E
.It is usually required that the function u(∙) is concave and nondecreasing, thus representing preferences of a riskaverse decision maker.
In this paper the portfolio optimization, we shall consider stochastic dominance relations between random returns in (2.1) if to avoid placing the decision vector, x, in a subscript expression, we shall simply write
)
(
)
;
(
h
x
F
R (x )h
F
=
and
F
2(
h
;
x
)
=
F
2 R ( x )(
h
)
. We say that portfoliox
dominates portfolio
y
under theFSDrules, if)
);
(
(
)
);
(
(
R
x
h
F
R
y
h
F
£
for all
h
Î
R
,where at least one strict inequality holds. Similarly, we say that
x
dominatesy
under the SSD rules))
(
)
(
(
R
x
f
SSD
R
y
, if)
);
(
(
)
);
(
(
22
R
x
h
F
R
y
h
F
£
for all
h
Î
R
,with at least one inequality strict. Recall that the individual returns
R
j have finite expected values and thus the functionF
2
(
R
(
x
);
×
h
)
is well defined.Stochastic dominance relations are of crucial importance for decisions theory. It is known that
)
(
)
(
x
R
y
R
f
FSD if and only if
(2.4)
E
[
u
(
R
(
x
))]
³
E
[
u
(
R
(
y
))]
for any nondecreasing function u(∙) for which these expected values are finite. Furthermore,
)
(
)
(
x
R
y
R
f
SSD if and only if (2.4) holds true for every nondecreasing and concave u(∙) for which
these expected values are finite [4, 5, 8].
A portfolio x is called SSDefficient (or FSDefficient) in a set of portfolios
X
if there is noy
Î
X
such that(
R
(
y
)
f
SSD
R
(
x
))
(or(
R
(
y
)
f
FSD
R
(
x
))
).We shall focus our attention on the SSD relation, because of its consistency with riskaverse preferences: if
(
R
(
y
)
f
SSD
R
(
x
))
, then portfolio x is preferred to y by all riskaverse decision makers.The starting point for our model is the assumption that a reference random return Y having a finite expected value is available. It may be an index or our current portfolio. Our intention is to have the return of the new portfolio,
R
(
x
)
, preferable over Y. Therefore, we introduce the following optimization problem [4]:(2.9)
max
x
f
(
)
(2.10) Subject toR
(
f
x
)
( 2 )
Y
,Here
f
:
X
®
R
is a concave continuous functional. In particular, we may use)]
(
[
)
(
x
R
x
f
=
E
and this will still lead to nontrivial solutions, due to the presence of the dominance constraint
Proposition 2.1 [4] Assume that
Y
has a discrete distribution with realizationsy
i,
=
i
1
,
m
. Then relation(2.10)is equivalent to(2.12)
E
[(
y
i-
R
(
x
))
+]
£
E
[(
y
i-
Y
)
+],
"
i
=
1
,
m
.Let us assume now that the returns have a discrete joint distribution with realizations
r
jt ,t ,
=
1
T
,n
j ,
=
1
, attained with probabilitiesp
t ,
t ,
=
1
T
. The formulation of the stochastic dominance relation (2.10) resp. (2.12) simplifies even further. Introducing variabless
itrepresenting shortfall of R(x) belowyi in realization t,
i ,
=
1
m
andt ,
=
1
T
, we obtain the following result.Proposition 2.2 The problem (2.9)(2.11) is equivalent to the problem:
(2.13)
max
x
f
(
)
(2.14) Subject to n j it i
j jt
y
s
x
r
-
£
-
-
å
=1
,
i ,
=
1
m
,t ,
=
1
T
,(2.15) 2
(
;
)
1 it i
T
t
p
ts
F
Y
y
£
å
=,
i ,
=
1
m
,(2.16)
s
it
³
0
,i ,
=
1
m
,t ,
=
1
T
(2.17)
å
=
£
n
j j
x
1
1
(2.18)
å
=
-
£
-
n
j j
x
1
1
(2.19)x
j³
0
,j ,
=
1
n
, and problema (2.13)–(2.19) can be written as(2.20)
max
X
j
(
)
=å
= n
j j j
X
c
1
max
(2.21) Subject to n mT i
j ij j
b
X
a
£
å
+ =1,
i
=
1
,
mT
+
m
+
2
,(2.22)
X
j³
0
,j
=
,
1
n
+
mT
, WhereX
=
(
x
1,...,
x
n,
s
11,...,
s
1 T,
s
21,...,
s
2 T,...,
s
m 1,...,
s
mT)
ï
î
ï
í
ì
+
-
+
+
=
-
=
+
+
=
-
-
=
+
+
=
=
-
=
otherwise
i
T
n
n
j
and
T
K
m
K
Km
i
T
K
m
K
Km
i
n
j
r
a
ij
ij
,
0
1
)
1
(
,
1
,
)
1
(
,
0
,
)
1
(
,
1
,
1
)
1
(
,
0
,
)
1
(
,
1
,
,
1
,
î
í
ì
=
=
+
=
otherwise
mT
i
n
j
a
ij,
0
1
,
î
í
ì
-
=
=
+
=
otherwise
mT
i
n
j
a
ij,
0
2
,
,
1
,
1
î
í
ì
=
+
-
+
+
=
=
+
+
+
=
- - -otherwise
m
mT
mT
i
m
K
TK
n
K
T
n
j
p
a
j n T K ij,
0
2
,
3
,
,
1
,
,
1
)
1
(
,
) 1 (
In the next section we extended this result to fuzzy decisions theory.
3. Portfolio problems with fuzzy technological coefficients In this section presents an approach to portfolio selection using fuzzy decisions theory.
We consider a linear programming problem (2.20) – (2.22) with fuzzy technological coefficients [13].
(3.1)
max
X
j
(
)
=å
= n
j j j
X
c
1
max
(3.2) Subject to n mT i
j ij j
b
X
a
£
å
+ =1~
,i
=
1
,
mT
+
m
+
2
,(3.3)
X
j³
0
,j
=
,
1
n
+
mT
.Assumption 3.1.
a
~
ij is a fuzzy number for any i and j. In this case we consider the following membership functions: (i) 1. Fori
=
Km
+
1
,
(
K
+
1
)
m
,
K
=
0
,
(
T
-
1
)
andj ,
=
1
n
ï
î
ï
í
ì
+
-
³
+
-
<
£
-
-
+
-
-
<
=
.
0
,
/
)
(
,
1
)
(
ij ij
ij ij ij
ij ij
ij
ji
a
d
r
t
if
d
r
t
r
if
d
t
d
r
r
t
if
t
ij
m
2. For
i
=
Km
+
1
,
(
K
+
1
)
m
,
K
=
0
,
(
T
-
1
)
and j=n+T(iKm1)+K+1ï
î
ï
í
ì
+
-
³
+
-
<
£
-
-
+
-
-
<
=
,
1
0
,
1
1
/
)
1
(
1
1
)
(
ij ij ij
ij a
d
t
if
d
t
if
d
t
d
t
if
t
ij
m
(ii) For
i
=
mT
+
3
,
mT
+
m
+
2
,K ,
=
1
m
andj
=
n
+
T
(
K
-
1
)
+
1
,
n
+
TK
ï
î
ï
í
ì
+
³
+
<
£
-
+
<
=
- - -
- - - -
- -
- - -
- - -
, )
1 (
) 1 (
) 1 ( )
1 (
) 1 (
0
,
/
)
(
,
1
)
(
ij
ij ij
ij K T n j a
d
p
t
if
d
p
t
p
if
d
t
d
p
p
t
if
t
K T n j
K T n j K
T n j
K T n j ij
m
For defuzzification of this problem, we first fuzzify the objective function. This is done by calculating the lower and upper bound of the optimal values first. The bounds of the optimal values
z
land
z
u are obtained by solving the standard linear programming problems
(3. 4)
z
1=
max
j
(
X
)
(3. 5) Subject to n mT j i
j ij
b
X
a
£
å
+ =1,
i
=
1
,
mT
+
m
+
2
(3.6)
X
j³
0
,j
=
,
1
n
+
mT
and
(3.7)
z
2=
max
j
(
X
)
(3.8) Subject to n mT j i
j ij
b
X
a
£
å
+ =1ˆ
,i
=
1
,
mT
+
m
+
2
(3.9)X
j³
0
,j
=
,
1
n
+
mT
where
ï
î
ï
í
ì
-
=
+
-
+
+
=
+
+
=
+
-
-
=
+
+
=
=
+
-
=
otherwise
d
T
K
and
i
T
n
n
j
m
Km
Km
i
d
T
K
and
m
K
Km
i
n
j
d
r
a
ij ij
ij ij
ij
,
)
1
(
,
0
,
1
)
1
(
,
1
,
)
1
(
,
1
,
1
)
1
(
,
0
)
1
(
,
1
,
,
1
,
ˆ
ï
î
ï
í
ì
+
=
=
+
=
otherwise
d
mT
i
n
j
d
a
ij ij
ij
,
1
,
,
,
1
,
1
ˆ
ï
î
ï
í
ì
-
+
=
=
+
=
otherwise
d
mT
i
n
j
d
a
ij ij
ij
,
2
,
,
1
,
1
ˆ
The objective function takes values between
z
1and
z
2 while technological coefficients vary between
a
ij anda
ij+
d
ij . Letz
l=
min(
z
1,
z
2)
andz
=
umax(
z
1,
z
2)
. Thenz
l and
z
u are called the lower and upper bounds of the optimal values, respectively.
Assumption 3.2. The linear crisp problem (3.4)(3.6) and (3.7)(3.9) have finite optimal values. In this case the fuzzy set of optimal values,G, which is subset of
R
n , is defined as [10]
(3.10)
ï
ï
ï
î
ï
ï
ï
í
ì
³
£
£
-
-
<
=
å
å
å
å
= = =
=
n
j j j
n
j j j
n
j j j
n
j j j
G
z
X
c
if
z
X
c
z
if
z
z
z
X
c
z
X
c
if
X
1 1 1
1
1
)
/(
)
(
0
)
(
u u l
l u l
l
m
The fuzzy set of theith constraint,
C
i , which is a subset of
R
n + mT , is defined by:(3.11)
m
Ci(
X
)
=
ï
ï
ï
î
ï
ï
ï
í
ì
+
-
³
+
-
<
£
-
+
-
<
å
å
å
å
å
å
= = = = = = nj ji ij j
i n
j
n
j ij ij j
i j ij n j n
j ij j j
ij i
n
j ji j i
X
d
r
b
X
d
r
b
X
r
X
d
X
r
b
X
r
b
1 1 1 1 1 1
)
(
,
1
)
(
,
/
)
(
,
0
2. For
i
=
Km
+
1
,
(
K
+
1
)
m
andK
=
T
0
,
(
-
1
)
(3.12)
m
Ci(
X
)
=
ï
ï
ï
î
ï
ï
ï
í
ì
+
-
³
+
-
<
£
-
+
-
<
å
å
å
å
å
å
+ = + = = + + = = + = ) , ( 1 ) , ( 1 ) , ( 1 ) , ( 1 1 ) , ( 1)
1
(
,
1
)
1
(
,
/
)
(
,
0
K i n n
j ij j
i K i n n j K i n n
j ij j
i j K i n n j n
j ij j j i K i n n j j i
X
d
b
X
d
b
X
X
d
X
b
X
b
wheren(i,K)=n+T(iKm1)+K+1
(ii) For
i
=
mT
+
3
,
mT
+
m
+
2
,
andK ,
=
1
m
(3.13)
m
Ci
(
X
)
=ï
ï
ï
î
ï
ï
ï
í
ì
+
³
+
<
£
-
<
å
å
å
å
å
å
+ - + = - - - + - + = + - + = - - - - - - + - + = + - + = - - - + - + = - - - TK n K T nj j n T K ij j i TK n K T n j TK n K T n
j j n T K ij j i j K T n j TK n K T n j TK n K T n j ij j j K T n j i TK n K T n
j j n T K j i
X
d
p
b
X
d
p
b
X
p
X
d
X
p
b
X
p
b
) 1 ( ( 1 ) )
1
( ( 1 ) ( 1 ) ( 1 ) )
1
( ( 1 ) ( 1 )
) 1 ( ( 1 )
.
)
(
,
1
,
)
(
,
/
)
(
,
,
0
By using the definition of the fuzzy decisions proposed by Bellman and Zadeh [1], we have)))
(
(
min
),
(
min(
)
(
X
GX
j C jX
D
m
m
m
=
.
i.e.
)))
(
(
min
),
(
min(
max
))
(
(
max
00 D
X
x GX
j C jX
x³
m
=
³
m
m
.Consequently, the problem (3.1)(33) becomes to the following optimization problem
(3.14)
max
l
(3.15)
m
G(
X
)
³
l
(3.16)
m
Ci
(
X
)
³
l
,
i
=
1
,
mT
+
m
+
2
, (3.17)X
j³
0
,0
£
l
£
1
,j ,
=
1
mT
.By using (3.10) and (3.11)(3.13), we obtain the following theorem.
(3. 18)
max
l
(3.19)
(
)
20
1 2
1
-
-
å
+
£
=
z
X
c
z
z
nj j j
l
(3.20)
å
+=
£
-
mT n
j ij j i
b
X
a
1
0
)
(
ˆ
l
,
i
=
1
,
mT
+
m
+
2
, (3.21)X
j³
0
,0
£
l
£
1
,j ,
=
1
mT
.where
ï î ï í ì
+ - + = -
= +
+ = +
-
- = +
+ = = +
- =
otherwise, ,
, 1 ) 1 ( , 1 )
1 ( , 0 , ) 1 ( , 1 ,
1
, ) 1 ( , 0 ,
) 1 ( , 1 ,
, 1 , )
( ˆ
ij ij
ij ij
ij
d
i T n j and T
K m K Km i d
T K and m K Km i n j d r a
l
l
l
l
ï
î
ï
í
ì
+
=
=
+
=
,
otherwise
,
,
1
,
,
1
,
1
)
(
ˆ
ij ij
ij
d
mT
i
n
j
d
a
l
l
l
ï
î
ï
í
ì
-
+
=
=
+
=
otherwise,
,
,
2
,
,
1
,
1
)
(
ˆ
ij ij
ij
d
mT
i
n
j
d
a
l
l
l
ï
î
ï
í
ì
+
=
+
-
+
+
=
+
+
+
=
- - -.
otherwise
,
2
,
3
,
,
1
)
1
(
,
)
(
ˆ
( 1 )ij
ij K
T n j
ij
d
m
mT
mT
i
TK
n
K
T
n
j
d
p
a
l
l
l
Notice that, the constraints in problem (3.18)(3.21) containing the cross product term
l
X
j are not convex. Therefore the solution of this problem requires the special approach adopted for solving general nonconvex optimization problem.4. Solution of defuzzificated problems
In this section, we present the modified subgradient method [6] and use it for solving the defuzzificated problems (3.18)(3.21) for nonconvex constrained problems and can be applied for solving a large class of such problems.
Notice that, the constraints in problem (3.18)(3.21) generally are not convex. These problems may be solved either by the fuzzy decisive set method, which is presented by Sakawa and Yana [15], or by the linearization method of Kettani and Oral [2].
4.1. Application of modified subgradient method to fuzzy linear programming problems.
For applying the subgradient method [6] to the problem (3.18)(3.21), we first formulate it with equality constraints by using slack variables
y
0and
y
i ,
i
=
1
,
mT
+
m
+
2
. Then, we can be written as(4.1)
max
l
(4.2)
(
,
,
)
(
)
2 00
1 2 1 0
0
=
-
-
å
+
+
=
=
y
z
X
c
z
z
y
X
g
nj j j
l
l
(4.3)
g
i(
X
,
l
,
y
i)
=
å
+=
=
+
-
mT n
j ij j i i
y
b
X
a
1
(
)
0
ˆ
l
,
i
=
1
,
mT
+
m
+
2
For this problem the set Scan be defined as
}
1
0
,
0
,
0
)
,
,
{(
³
³
£
£
=
X
p
l
X
y
l
S
.Since
max
l
=
-
min(
-
l
)
andg
=
(
g
0,...,
g
mT + m + 2)
the augmented Lagrangian associated withthe problem (4.1)(4.4) can be written in the form
. 2
1 ˆ ( )
0 2 1 ) 2 1 ( 0 2 1 2 2
1 ˆ ( )
2 0 2 1 ) 2 1 ( ) , , ( 1 1 å + + = ÷ ø ö ç è æ - + å -÷ ÷ ø ö ç ç è æ + å = + - - - ú ú ú û ù ê ê ê ë é å + + = ÷ ø ö ç è æ å - + + ï þ ï ý ü ï î ï í ì + + å = - - + - = + = + = m mT
i u a X b y y
z n
j c j Xj z
z
m mT
i a X b y y
z n
j c j Xj z z c c u x L i i j mT n j ij i i i j mT n j ij l l m l l l The modified subgradient method may be applied to the problem (4.1)(4.4) in the following way: Initialization Step.Choose a vector
(
u
1 0,
u
1 1,...,
u
mT1 + m + 2,
c
1)
withc
1
³
0
. Letk
=
1
, and go to mainstep.
Main Step.
Step 1.Given
(
u
0k,
u
1 k,...,
u
mT k + m + 2,
c
k)
; solve the following subproblem :. 2
1 1 ˆ ( )
0 2 1 ) 2 1 ( 0 2 1 2 2
1 1 ˆ ( )
2 0 2 1 ) 2 1 ( min å + = ÷ ÷ ø ö ç ç è æ + - å + = -÷ ÷ ø ö ç ç è æ + å = + - - - ú ú ú û ù ê ê ê ë é å + + = ÷ ÷ ø ö ç ç è æ + - å + = + ï þ ï ý ü ï î ï í ì + + å = - - + - mT
i X j b i y i mT
n j a ij i
u y
z n
j c j Xj z
z u
m mT
i b i y i mT
n
j a ij X j y
z n
j c j Xj z z c l l l l l
.
)
,
,
(
X
y
l
Î
S
Let
(
X
k,
y
k,
l
k
)
be any solution. Ifg
(
X
k,
y
k,
l
k
)
, then stop;(
u
0k,
u
1 k,...,
u
mT k,
c
k)
is a solution to dual problem,(
X
l
k,
k
)
is a solution to problem (3.18)(3.21). Otherwise, go to Step 2.Step 2.Let
÷
÷
ø
ö
ç
ç
è
æ
+
+
-
-
-
=
å
= + 0 2 1 2 1 0 10
u
h
(
z
z
)
c
x
z
y
u
nj j j k k k
l
÷
÷
ø
ö
ç
ç
è
æ
+
-
-
=
å
=+ n
j ij j i i
k k i k
i
u
h
a
X
b
y
u
1
1
ˆ l
(
)
,i
=
1
,
mT
+
m
+
2
)
,
,
(
)
(
1 k k k k k k
k
c
h
g
X
y
c
+=
+
+
e
l
where
h
k and
e
k are positive scalar stepsizes and
h
e
k>
k
>
0
, replacek byk + 1;and repeat Step 1.4.2. The algorithm of the fuzzy decisive set method
Algorithm
Step 1. Set
l
= 1 and test whether a feasible set satisfying the constraints of the problem (3.18)(3.21) exists or not using phase one of the simplex method. If a feasible set exists, setl
= 1: Otherwise, set
0
=
L
l
and
l
R
=
1
and go to the next step.Step 2. For the value of
l
=
(
l
L+
l
R)
/
2
; update the value ofl
L and
l
R using the bisection method as follows :
l
l
=
Lif feasible set is nonempty for
l
l
l
=
Rif feasible set is empty for
l
.Consequently, for each
l
, test whether a feasible set of the problem (3.18)(3.21) exists or not using phase one of the Simplex method and determine the maximum valuel
*satisfying the constraints of the problem (3.18)(3.21).
References
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, A
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