Associativity of triangular norms in light of web geometry

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1 Associativit of triangular norms in light of web geometr Milan Petrík 1,2 Peter Sarkoci 3 1. Institute of Computer Science, Academ of Sciences of the Czech Republic, Prague, Czech Republic 2. Center for Machine Perception, Department of Cbernetics Facult of Electrical Engineering, Czech echnical Universit Prague, Czech Republic 3. Department of Knowledge-Based Mathematical Sstems, Johannes Kepler Universit, Linz, Austria s: {petrik@cs.cas.cz petrikm@cmp.felk.cvut.cz}, peter.sarkoci@{jku.at gmail.com} Abstract he aim of this paper is to promote web geometr and, especiall, the Reidemeister closure condition as a powerful and intuitive tool characterizing associativit of the Archimedean triangular norms. In order to demonstrate its possible applications, we provide the full solution to the problem of conve combinations of nilpotent triangular norms. Kewords Archimedean triangular norm, web geometr, Reidemeister closure condition. 1 riangular norms he notion of triangular norm was originall introduced within the framework of probabilistic metric spaces [12]. Since then, triangular norms have found diverse applications in the theor of fuzz sets, fuzz decision making, in models of certain man-valued logics or in multivariate statistical analsis; for a reference see the books b Alsina, Frank, and Schweizer [5] and b Klement, Mesiar, and Pap [8]. A conjunctor is a function K : [0, 1] 2 [0, 1] which is nondecreasing in both arguments, commutative, and which satisfies the boundar condition (, 1 = for all [0, 1]. A triangular norm (shortl a t-norm, usuall denoted b is a conjunctor which satisfies the associativit equation ( (,,z= (, (, z for all,, z [0, 1]. his paper deals mainl with Archimedean t-norms. Let us recall that for ever t-norm, a number n N {0},and [0, 1] a natural power of, denoted ( (n, is defined b: { 1 ( (n ( if n =0, =, ( (n 1 (1 if n>0. A t-norm is said to be Archimedean if and onl if for ever pair, ]0, 1[, <, there eists a natural number n N such that ( (n <. A t-norm which is continuous and strictl increasing on the half-open square ]0, 1] 2 is said to be strict. A continuous t-norm is called nilpotent if and onl if for ever ]0, 1[ there eists a natural number n N such that ( (n =0. A prototpical eample of a strict and a nilpotent t-norm is the product t-norm, P (, =, and the Łukasiewicz t-norm L (, =ma{ + 1, 0}, Figure 1: Eample of a complete 3-web. respectivel. It is possible to show that a continuous Archimedean t-norm is either strict or nilpotent. A t-norm is said to be cancellative if it satisfies (a, b = (a, c b = c for ever a, b, c [0, 1], a 0. A t-norm is said to be weakl cancellative [9] if it satisfies (a, b = (a, c b = c for ever a, b, c [0, 1] with (a, b 0and (a, c 0. Ever cancellative t-norm is weakl cancellative. Under the assumption of continuit, the set of cancelative t-norms and the set of strict t-norms coincide. Under the same assumption, the set of weakl cancelative t-norms coincides with the set of Archimedean t-norms. 2 Web geometr and local loops In this section, web geometr is eplained as a tool allowing to visualize algebraic identities. In particular, it is shown that it visualizes associativit of Archimedean t-norms. A detailed introduction to the subject is given in the monograph b Blashke and Bol [6]. Also the collection of papers b Aczél, Akivis, and Goldberg [1, 2, 3] can serve as an (English introductor tet. Definition 2.1 A groupoid (or a magma is an algebraic structure G =(G, on a set G where : G G G is a binar operation. A quasigroup is a groupoid in which the 595

2 equations a = b and a = b have unique solutions for ever a and b in G. Finall, a loop is an algebraic structure L =(G,,e where (G, is a quasigroup and e is an identit element, i.e. e = = e for ever G. he definition of a quasigroup, G =(G,, allows to define the left and the right inverse. For this purpose we introduce a prefi notation g(, =.he left inverse is then defined for ever u G as 1 g(u, = such that g(, =u. Similarl, the right inverse is, for ever v G, g 1 (, v = such that g(, =v. We sa that g is invertible in, resp.. Defining a point A =(a, b G G we ma introduce a new operation, : G G G, as u v = g ( 1 g(u, b,g 1 (a, v. (2 It is eas to show that (G, is a loop with a unit element e = g(a, b; this loop is called a local loop of the quasigroup (G, at the point A =(a, b. Definition 2.2 Let M be a non-empt set and let λ 1,λ 2,λ 3 be three families of subsets of M. oelementsofm we refer as to points and to elements of λ α, α {1, 2, 3}, as to lines. Wesathatthesstem(M,λ 1,λ 2,λ 3 is a complete 3-web if and onl if the following conditions hold: g u g U A W V u w = u v v Figure 2: Operation defined on a complete 3-web. g g v 1. An point p M is incident to just one line of each famil λ α, α {1, 2, 3}. 2. An two lines of different families are incident to eactl one point of M. 3. wo distinct lines from the same famil λ α are disjoint. Note that Item 3 of the definition is redundant and can be derived from Item 1. We kept the definition in this redundant form in order to keep some of the furher considerations simpler. Usuall M is equipped with a topolog which turns the set into a manifold. In such a case the families λ α are often required to be foliations. Figure 1 shows such an eample of 3-web where M is a two dimmensional domain in plane and λ α are foliations of co-dimension 1. Notice that all the eamples of 3-webs, shown in figures of this paper, are simplified. hat means that onl some particular lines from uncountable sets λ α, α {1, 2, 3}, are drawn. Complete 3-webs are closel connected to quasigroups and, especiall, loops. Ever quasigroup defines a 3-web in the following wa. Let G =(G, be a quasigroup defined on a manifold M = G G and let, G. Let the lines of sets λ 1, λ 2,andλ 3 be given b the equations = a, = b, and = c respectivel for fied a, b, c G. he pair (M,λ α, α {1, 2, 3}, is then a 3-web. Conversel, a 3-web (M,λ α, α {1, 2, 3},onamanifold M, defines a binar operation which is invertible with respect to both operands; et this time the result is not unique. Denote one set of the lines of the 3-web as λ, second as λ, and third as λ z (the lines in the set λ z are called contour lines, let A M and let g λ and g λ be the lines passing through Figure 3: 3-webs given b the product t-norm, P, and b the Łukasiewicz t-norm, L. A; cf. Figure 2. Now we define an operation : λ z λ z λ z. ake u, v λ z, define points U, V M as U = u g and V = v g, respectivel. Let g u λ be the line passing through U and let g v λ be the line passing through V. Denote the intersection of these two lines as W = g u g v ;the line w passing through W is the result of the operation u v. It can be shown easil that the operation is invertible in both variables. Moreover, the line e λ z, passing through the point A, behaves, with respect to the oparation, as the unit element. hus (λ z, is a loop. Moreover, this loop coincides, up to an isomorphism, with the local loop of (G, at the point A. -norms, defined on the unit interval [0, 1], form neither groupoids nor loops. Nevertheless, the 3-web given b a continuous Archimedean (i.e. strict or nilpotent t-norm satisfies all the requirements given b Definition 2.2 if the manifold M is defined as the subset of [0, 1] 2 where the t-norm attains non-zero values. A manifold, induced this wa b a binar operation : [0, 1] 2 [0, 1], will be denoted as Man. hus { } M =Man = (, [0, 1] 2 (, > 0. (3 Figure 3 shows 3-webs given b P and L as eamples of a strict and a nilpotent t-norm respectivel. 596

3 b 4 4 c bc (abc a(bc Figure 4: Eample of a closed Reidemeister figure and a nonclosed Reidemeister figure. 3 Reidemeister closure condition Different tpes of 3-webs are characterized b closure conditions. hese closure conditions have their counterparts in the related loops as algebraic properties of the loop operations. In this tet we are interested in the associativit of t-norms since the associativit is the onl propert of a t-norm which cannot be intuitivel interpreted from its graph. he 3-web counterpart of the associativit is the Reidemeister closure condition. A 3-web satisfies the Reidemeister closure condition if and onl if ever Reidemeister figure in this web is closed; see Figure 4. Described in the terminolog of contour lines, the Reidemeister closure condition is as follows. Let 1, 2, 3, 4 λ and 1, 2, 3, 4 λ. If the points ( 1 2 and ( 2 1 lie on the same contour line (i.e. a line from the set λ z, and if so do the pair of points ( 1 4 and ( 2 3, and the pair of points ( 3 2 and ( 4 1, then the points ( 3 4 and ( 4 3 also lie on the same contour line. he left part of Figure 4 shows a 3-web which satisfies the Reidemeister closure condition whereas the right part shows a 3- web where the condition is violated. Let us now define a relation M M. We sa that two points A, B M are in the relation, and we write A B, if and onl if the are both elements of the same line from the set λ z. Using the language of the Reidemeister condition reads as: ( ( (1 2 ( 2 1 & ( ( 1 4 ( 2 3 & ( ( 3 2 ( 4 1 ( ( 3 4 ( 4 3. (4 4 Reidemeister closure condition and continuous Archimedean t-norms We are going to show that the level set sstem of a continuous Archimedean t-norm alwas satisfies the Reidemeister closure condition. Moreover, the Reidemeister closure condition characterizes the associativit of a continuous Archimedean t-norm. For this purpose we recall that if we rela the requirement of the associativit in the definition of a t-norm, we obtain the definition of a conjunctor. Let K be a continuous Archimedean conjunctor; for the sake of compactness we will use the infi notation: K(, = ab Figure 5: Reidemeister figure on the 3-web given b a continuous Archimedean t-norm a b 2 3 Figure 6: Illustration for the proof: an arbitrar Reidemeister figure.. Let(M,λ α be the corresponding 3-web defined on the manifold M =ManK = {(, [0, 1] 2 >0}. Such a 3-web, as in the case of the continuous Archimedean t-norms, satisfies all the requirements given b Definition 2.2. In view of the previous section, the 3-web given b satisfies the Reidemeister closure condition if and onl if the following condition holds for an 1, 2, 3, 4, 1, 2, 3, 4 M: ( (1 2 = 2 1 & ( 1 4 = & ( 3 2 = 4 1 ( 3 4 = 4 3. (5 In the sequel, this condition will be denoted b R. Figure 5 shows that R implies the associativit of in a rather intuitive wa. For an a, b, c [0, 1] 2, such that (a b c Man and a (b c Man, there can be constructed a Reidemeister figure which, as can be seen in Figure 5, is closed if and onl if (a b c = a (b c. Figure 5 shows also that, in the other wa round, if is associative then all the Reidemeister figures that touch the lines given b =1and =1are closed. In other words, R has to be satisfied for an 2, 3, 4, 2, 3, 4 [0, 1] and, at least, for 1 = 1 =1. Let us denote this weaker condition b R 1. It is now clear that K is associative if and onl if it 597

4 a b 1 a b Figure 7: Illustration for the proof. c 1 Figure 9: Illustration for the proof. c Figure 8: Illustration for the proof. Figure 10: Illustration for the proof. satisfies R 1. In order to show that is associative if and onl if it satisfies R, we need to show that R 1 R. Obviousl, R R 1. he inverse implication, R 1 R, isgiventhe following wa. Let us have a continuous Archimedean conjunctor K which satisfies R 1. Let us have a Reidemeister figure drawn on its 3-web for arbitrar 1, 2, 3, 4, 1, 2, 3, 4 [0, 1], see Figure 6. We are going to show that this figure shall be alwas closed. hanks to R 1,the Reidemeister figures, shown in Figure 7-a and Figure 7-b, are closed. Combining these two figures together it can be concluded that the Reidemeister figure in Figure 8-c is closed as well. B the same deduction, from the closedness of the Reidemeister figures in Figure 9-a and Figure 9-b it can be concluded that the Reidemeister figure in Figure 10-c is closed. Now, combining the closed Reidemeister figures in Figure 8-c and Figure 10-c, the closedness of the Reidemeister figure in Figure 6 is proven. Corollar 4.1 A continuous Archimedean conjunctor is associative (and, thus, a t-norm if and onl if it satisfies R. 5 Applications 5.1 Problem of conve combinations of t-norms he question was introduced b Alsina, Frank, and Schweizer [4]. he approach of web geometr allows to give the following, recentl published [11], answers: heorem 5.1 Let 1 and 2 be two continuous Archimedean t-norms such that Man 1 Man 2. hen no non-trivial conve combination F of 1 and 2 is a t-norm. Corollar 5.2 Combining the result of heorem 5.1 with the result given b Jenei [7], a non-trivial conve combination of two distinct nilpotent t-norms is never a t-norm. Corollar 5.3 A non-trivial conve combination of a strict and a nilpotent t-norm is never a t-norm. his is an alternative proof of the result given earlier b Ouang, Fang, and Li [10]. Acknowledgment he first author was supported b the Grant Agenc of the Czech Republic under Project 401/09/H007. References [1] J. Aczél. Quasigroups, nets and nomograms. Advances in Mathematics, 1: , [2] M. A. Akivis and V. V. Goldberg. Algebraic aspects of web geometr. Commentationes Mathematicae Universitatis Carolinae, 41(2: , [3] M. A. Akivis and V. V. Goldberg. Local algebras of a differential quasigroup. Bulletin of the American Mathematical Societ, 43(2: , [4] C. Alsina, M. J. Frank, and B. Schweizer. Problems on associative functions. Aequationes Mathematicae, 66(1 2: , [5] C. Alsina, M. J. Frank, and B. Schweizer. Associative Functions: riangular Norms and Copulas. World Scientific, Singapore, [6] W. Blaschke and G. Bol. Geometrie der Gewebe, topologische Fragen der Differentialgeometrie. Springer, Berlin, [7] S. Jenei. On the conve combination of left-continuous t-norms. Aequationes Mathematicae, 72(1 2:47 59,

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