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Problems on associative functions. (English) Zbl 1077.39021
A function $$F:I \times I \to I$$, where $$I$$ is a real interval, is said to be associative if the following relation holds: $F(F(x,y),z)=F(x,F(y,z)), \quad x,y,z \in I.$ A special class of associative functions, the so-called $$t$$-norms, appears when studying probabilistic metric spaces. A $$t$$-norm is a function $$T$$ from the unit square to the unit interval, such that for all $$x,y,z,w \in [0,1]$$, \begin{aligned} &T(x,1)=x, \tag{a}\\ &T(x,y)=T(y,x), \tag{b}\\ &T(x,y) \leq T(z,w) \;\text{ whenever }\;x\leq z, \;y\leq w, \tag{c}\\ &T(T(x,y),z)=T(x,T(y,z)).\tag{d} \end{aligned} The authors studied extensively probabilistic metric spaces [see the book of B. Schweizer and A. Sklar, Probabilistic metric spaces, North-Holland Series in Probability and Applied Mathematics. New York-Amsterdam-Oxford: North-Holland. (1983; Zbl 0546.60010)] and consequently $$t$$-norms and related notions. In the present survey paper, after the presentation of the basic definitions, they propose 21 problems concerning associative functions. The paper contains a rich bibliography on the subject.

##### MSC:
 39B22 Functional equations for real functions 39B52 Functional equations for functions with more general domains and/or ranges 39-02 Research exposition (monographs, survey articles) pertaining to difference and functional equations 39B62 Functional inequalities, including subadditivity, convexity, etc. 62H05 Characterization and structure theory for multivariate probability distributions; copulas 60B05 Probability measures on topological spaces
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