Rahul Sharma (Editor)

Fuzzy subalgebra

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Fuzzy subalgebras theory is a chapter of fuzzy set theory. It is obtained from an interpretation in a multi-valued logic of axioms usually expressing the notion of subalgebra of a given algebraic structure.

Contents

Definition

Consider a first order language for algebraic structures with a monadic predicate symbol S. Then a fuzzy subalgebra is a fuzzy model of a theory containing, for any n-ary operation h, the axioms

x 1 , . . . , x n ( S ( x 1 ) . . . . . S ( x n ) S ( h ( x 1 , . . . , x n ) )

and, for any constant c, S(c).

The first axiom expresses the closure of S with respect to the operation h, and the second 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 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 d1,...,dn in D, if h is the interpretation of the n-ary operation symbol h, then

  • s ( d 1 ) . . . s ( d n ) s ( h ( d 1 , . . . , d n ) )
  • Moreover, if c is the interpretation of a constant c such that s(c) = 1.

    A largely studied class of fuzzy subalgebras is the one in which the operation coincides with the minimum. 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.

    Fuzzy subgroups and submonoids

    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

  • s(x) ≤ s(x−1).
  • It is possible to prove that the notion of fuzzy subgroup is strictly related with the notions of fuzzy equivalence. In fact, assume that S is a set, G a group of transformations in S and (G,s) a fuzzy 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 equivalence. Conversely, let e be a fuzzy equivalence in S and, for every transformation h of S, set

  • s(h)= Inf{e(x,h(x)): x∈S}.
  • Then s defines a fuzzy subgroup of transformation in S. In a similar way we can relate the fuzzy submonoids with the fuzzy orders.

    References

    Fuzzy subalgebra Wikipedia