criteria in determining an MST because there are
multiple attributes de®ned on each edge. The
problem may arise, for instance, when designing a
layout for telecommunication system, besides the
cost for connection between cities or terminals,
other factors such as the time for communication
or construction, the complexity for construction
and even the reliability are all important and have
to be taken into consideration. In all these cases,
the MST with multi-criteria is a more realistic
representation of the practical problem.
The multi-criteria Minimum Spanning Tree
(mc-MST) problem is not simply an extension of
MST from single objective to multiple objectives.
In general, we cannot get the optimal solution of
the problem because these multiple objectives
usually con¯ict with each other in practice. The
real solutions to this problem are a set of Pareto
optimal solutions [5] but the calculation of it is a
dicult task because it is an NP-hard problem and
no previous work in this area has been reported in
literature. A common solution to this problem is
to simplify multiple objectives into a single objec-
tive because those polynomial-time MST algo-
rithms are only eective to the MST with single
objective. But only one single-point solution in the
sense of Pareto optimality can be obtained, not to
mention the diculty in properly transforming the
multiple objectives into a single objective. The
decision maker in practice, however, may prefer
one (Pareto optimal) point over the others de-
pending on the situation. Consequently, it may be
useful to have (all) other possible Pareto optimal
solutions.
As one of the Evolutionary Computation (EC)
techniques, the GAs have been receiving great at-
tention and successfully applied in many research
®elds in the last decade [7,9,14], including the ef-
fective approaches on the multiobjective optimi-
zation problems [8,18,19,21]. In this paper we
propose a GA approach to deal with the mc-MST
problem. In keeping its network characteristic, we
adopt the Pr
ufer number as the tree encoding [17].
This encoding is capable of equally and uniquely
representing all possible spanning trees for a graph
and easily operated by GA approach. The pro-
posed GA approach can easily obtain a set of
Pareto optimal solutions instead of only one for
decision maker to choose. Combined with the
multiple criteria decision making (MCDM) tech-
nique [5], the proposed method can enforce the
GA approach to give out the Pareto optimal so-
lutions close to the ideal point as much as possible.
The ideal point is a solution satisfying all objec-
tives but not obtainable. Combined with the
nondominated sorting technique [21], the pro-
posed method can make the GA approach main-
tain all Pareto optimal solutions distributed along
the Pareto frontier, which gives out enough alter-
natives to re¯ect the achievements of each objec-
tive among multiple objectives. In order to verify
the eectiveness of the proposed GA approach, we
present an exact algorithm to give out all Pareto
optimal solutions for the mc-MST problem. The
numerical analysis shows that most of the results
got by the proposed GA approach are real Pareto
optimal solutions. Therefore, the proposed meth-
od in this paper is really eective to deal with the
mc-MST problem and capable of providing en-
ough alternatives for decision maker to choose.
The paper is organized as follows: In Section 2
the mc-MST problem is de®ned. Some techniques
in MCDM are described in Section 3. Section 4
discusses in detail our GA approach for the mc-
MST problem. In Section 5 the numerical experi-
ments on the proposed method are studied and the
conclusion follows in Section 6.
2. Problem description
Consider a connected undirected graph
G V ; E; where V fv
1
; v
2
; . . . ; v
n
g is a ®nite set
of vertices representing terminals or telecommu-
nication stations etc., and E fe
1
; e
2
; . . . ; e
m
g is a
®nite set of edges representing connections be-
tween these terminals or stations. Each edge has p
associated positive real numbers, representing p
attributes de®ned on it and denoted with
w
i
w
1i
; w
2i
; . . . ; w
pi
i 1; 2; . . . ; m. In practice
w
ki
k 1; 2; . . . ; p may represent the distance,
cost and so on.
Let x x
1
x
2
. . . x
m
be de®ned as follows.
x
i
1 if edge e
i
is selected;
0 otherwise:
142 G. Zhou, M. Gen / European Journal of Operational Research 114 (1999) 141±152