In regular and in intra-regular ordered semigroups the
ideals and the interior ideals coincide. In regular and in intra-regular
poe-semigroups the ideal elements and the interior ideal elements coincide.
In regular ordered semigroups the ideals and the interior ideals coincide.
This is the case for intra-regular ordered semigroups as well: In intraregular
ordered semigroups the ideals and the interior ideals are the same.
Suppose that the ordered semigroup possesses a greatest element, that is
it is a poe-semigroup. In regular poe-semigroups the ideal elements and
the interior ideal elements coincide. In intra-regular poe-semigroups the
ideal elements and the interior ideal elements coincide as well [7]. An
ordered semigroup S with a fuzzy subset dened on S, is called a fuzzy
ordered semigroup. The following question is natural: What happens in
case of fuzzy ordered semigroups? The theories of ordered semigroups
and of fuzzy ordered semigroups are parallel to each other. In this paper
we rst introduce the concept of a fuzzy interior ideal in an ordered
semigroup. Then, in an attempt to show the similarity between the
theory of ordered semigroups and the theory of fuzzy ordered semigroups,
we prove here that in regular and in intra-regular ordered semigroups the
concepts of fuzzy ideals and of fuzzy interior ideals are the same concepts.
Moreover we prove that for an ordered semigroup S, a set A is an interior
ideal of S if and only if the characteristic function fA is a fuzzy interior
ideal of S. We introduce the concept of a fuzzy simple ordered semigroup
and we prove that an ordered semigroup is simple if and only if it is
fuzzy simple. Finally, we characterize the simple ordered semigroups in
terms of fuzzy interior ideals. So in addition to the characterization of
simple ordered semigroups by means of ideals we already have, we obtain
characterizations of simple ordered semigroups in terms of fuzzy interior
ideals. Fuzzy interior ideals of semigroups (without order) and fuzzy
simple semigroups (without order) have been considered by Kuroki in
[9].
Given an ordered semigroup S, a fuzzy subset of S (or a fuzzy set in
S) is, by denition, an arbitrary mapping f : S ! [0; 1] where [0; 1] is
the usual closed interval of real numbers. For each subset A of S, the
characteristic function fA is the fuzzy subset on S dened as follows: