# Fuzzy subalgebra

In fuzzy logic given a first order language and a valuation structure *V*, we can interpret it by a *fuzzy interpretation*, i.e. a pair (D,I) such that D is a nonempty set and I, the *interpretation function* is a map associating any n-ary functor with an n-ary operation and any constant with an element of D (as in the classical case). Moreover, I associates any n-ary predicate name with a suitable n-ary fuzzy relation in D. The fuzzy subalgebras are particular fuzzy interpretations of a first order language for algebraic structures with a monadic predicate symbol S. The intended meaning of S(x) is that x is an element of the subalgebra. More precisely, a *fuzzy subalgebra*, is a model of a theory containing, for any *n*-ary operation name h, the axiom

A1 x_{1}..., x_{n}(S(x_{1})ʌ...ʌS(x_{n}) → S(h(x_{1},...,x_{n}))

and, for any constant c, the axiom

A2 S(c).

A1 expresses the closure of S with respect to the operation h, A2 expresses the fact that c is an element in S. As an example, assume that the valuation structure is defined in [0,1] and denote by the operation in [0,1] used to interpret the conjunction. Then it is easy to see that a fuzzy subalgebra of an algebraic structure whose domain is D is defined by a fuzzy subset s : D → [0,1] of D such that, for every d_{1},...,d_{n} in D, if **h** is the interpretation of the n-ary operation symbol h, then

i) s(d_{1})s(d_{n})≤ s(**h**(d_{1},...,d_{n}))

Moreover, if **c** is the interpretation of a constant c

ii) s(**c**) = 1.

In a largely studied class of valuation structures the product coincides with the joint operation. In such a case it is immediate to prove the following proposition.

**Proposition.** A fuzzy subset s of an algebraic structure defines a fuzzy subalgebra if and only if for every λ in [0,1], the closed cut {x D : s(x)≥ λ} of s is a subalgebra.

The *fuzzy subgroups* and the *fuzzy submonoids* are particularly interesting classes of fuzzy subalgebras. In such a case a fuzzy subset *s* of a monoid (M,•,**u**) is a fuzzy submonoid if and only if

1) s(u) =1 2) s(x)s(y) ≤ s(x•y)

where **u** is the neutral element in A. Given a group G, a *fuzzy subgroup* of G is a fuzzy submonoid s of G such that

3) s(x) ≤ s(x^{-1}).

The notions of fuzzy submonoid and fuzzy subgroup are strictly related with the notions of * fuzzy order* and *fuzzy equivalence*, respectively.

**Proposition**. Assume that S is a nonempty set, G a group of transformations in S and (G,s) a fuzzy submonoid (subgroup) of G. Then, by setting

e(x,y) = Sup{s(h) : h is an element in G such that h(x) = y}

we obtain a fuzzy order (a fuzzy equivalence). Conversely, let e be a fuzzy order (equivalence) in S and define s by setting, for every transformation h of S,

s(h)= Inf{e(x,h(x)): xS}.

Then s defines a fuzzy submonoid (fuzzy subgroup) of transformation in S.

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