Abstract-We study two categories, both having intervalvalued fuzzy sets as objects. One has certain functions between the domains as morphisms and the other is expanded to include certain relations between the domains as morphisms. We describe some of the basic properties of each of these categories. We lift tnorms and negations to the category with relations. The t-norms are used to define a "tensor product" on that category.
I. CATEGORIES OF FUZZY SETSA category consists of objects, morphisms, a composition rule for morphisms, and an identity morphism for each object. Some examples of categories are vector spaces with linear transformations; sets with functions; topological spaces with continuous functions; and groups with homomorphisms. In all these examples, composition is ordinary composition of functions. Another example, which we shall refer to, is the less familiar category having sets as objects and relations as morphisms, and using ordinary composition of relations.The study of categories of fuzzy sets with the truth-value algebra I = ([0, 1] , ∧, ∨, ¬, 1, 0), where the operations ∧ and ∨ are the usual min and max, negation ¬x = 1 − x, and constants 1 and 0, has a long history. See, for example, [2], [5], [7], [8], [9]. We extend this to categories of interval-valued fuzzy sets with functions or relations, and lift elements of the structure on the truth-value algebra to the categorical setting.The categories of fuzzy sets that we consider have as objects interval-valued fuzzy sets A, with is the truth-value algebra for interval-valued fuzzy sets. Some of the basic properties of this algebra may be found in [4]. Since the domains X, Y, ... of the fuzzy sets need to be identified in order to describe the morphisms between objects, and the range I[2] of the fuzzy sets will be always the same, we will write (X, A) or (Y, B) for objects X. Morphisms for these categories will be functions or relations satisfying the conditions described in 1) and 2), and composition is in both cases ordinary composition of functions or relations.[2] is a distributive lattice, in fact, a De Morgan algebra since the negation is an involution satisfying the De Morgan laws [1].We will consider two categories, FIV and FIVR, with objects interval-valued fuzzy sets and morphisms functions and relations, respectively:1) The objects of FIV are functions (X, A) = X A → I[2] , and the morphismsComposition is ordinary composition of functions and the identity id X :2) The objects of FIVR are functions (X, A) = X A → I [2] , and the morphismsComposition is ordinary composition of relations and the identity id X : X → X is the relation id X = {(x, x) : x ∈ X}. The inequality conditions in 1) and 2) will be indicated by diagrams of the form(where R could be a function or relation). For purposes of comparison, we will also consider the categories Set (the objects are sets and the morphisms are functions) and Rel (the objects are sets and the morphisms are relations R ⊆ X × Y ). We show that morphisms are closed under composition in FIVR.Lem...