option[21], robust multi-stage investment [4, 49], power supply[46], truss topology design[1],
supply chain management[9, 16], Other application of this methodology can be found in
[48, 20], etc. For complete survey of robust optimization, we refer the author to [12].
Though robust methodology is much useful, there a r e some drawbacks for t his method-
ology. Firstly, to use robust optimization methodology, one needs to know exactly the
uncertainty set U before modelling, which is generally impossible, while exactness of the
uncertainty set affects the robust solutions terribly. In practice, one can only know that
a uncertain parameter belongs to some set fuzzily. Secondly, robust optimization method-
ology is too pessimistic. It considers all possible uncertainty equally, while in practice,
some uncertainty will just happen with small probability, which may influences robust so-
lutions and values vastly. Moreover, it’s impossible for one to pretend against all po ssible
uncertainty in practice because uncertainty have multiple origins[2]. These two drawbacks
require us to generalize robust optimization methodology.
We extend robust optimization methodology via introducing fuzziness to the uncer-
tainty set U considering the fact that it can not be known exactly. We do so also because
fuzziness is used in various fields when modelling since Zedah [34] posed t he concepts of
fuzzy numbers and fuzzy set. Currently, the concept of fuzzy programming is posed and
it has been applied in various kinds of real problems, such as water allocation, facility
location, etc. In reality, data is always considered to be fuzzy instead o f specific. In our
generalization of robust optimization methodology, the uncertain parameter s is assumed
to belong to an uncertainty set fuzzily, correspondingly, the constraints are not pro hibited
to hold exactly, instead, they a r e just requested to hold vaguely. By dosing so, we get
a new kind of optimization problems, called robust fuzzy optimization problems in the
sequel. A robust fuzzy optimization problem can be formulated as follows.
min c
T
x (1.3)
s.t. g(x, s) ∈
˜
S, ∀s ∈
˜
U
where
˜
S and
˜
U a r e two fuzzy sets.
Obviously, as s is a fuzzy number, g(x, s) is a f uzzy vector. In the above formulation,
for any fuzzy number s ∈
˜
U, the solution x should satisfy that g(x, s) is in the fuzzy set
˜
U, which embodies robustness. Also, we consider the data s to be fuzzy, which incarnates
fuzziness. Though the terminology ”robust fuzzy” appeared already in control theory[30],
robust design[29], clustering[31], in which we emphasize that the idea adopted in [30] is
something like the one posed in this paper, which introduces the way to keep a fuzzy
PID control structure in another fuzzy PID structure(essentially robust fuzziness), the
idea of robust fuzziness in this article is posed in a more general formulation, which is a
generalization of [14].
The structure of the rest ar t icle is as follows. In section 2, we come to introduce some
basic concepts of fuzzy sets and membership functions. In section 3, we make a basic
assumption for problem (1.3), and then show that robust fuzzy optimization problems
are generally difficult. We design a sampling algorithm to solve the robust fuzzy convex
optimization problems in section 4. Finally, in section 5, future r esearch on this topic is
remarked.
2 Preliminaries
In this section, we introduce some basic concepts and results on fuzzy sets and mem-
bership functions. All these concepts and results can be fo und in [10, 11, 2 8], etc.
3
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