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On two types of discrete implications. (English) Zbl 1084.03021
Summary: This paper deals with two kinds of implications defined from t-norms, t-conorms and strong negations on a finite chain $$L$$: those defined through the expressions $$I(x, y) = S(N(x), T(x, y))$$ and $$I(x, y) = S(T(N(x), N(y)), y)$$. They are called QL-implications and NQL-implications respectively. We mainly study those QL- and NQL-implications derived from smooth t-norms and smooth t-conorms. It is characterized when functions defined in these ways are implication functions, and their analytical expressions are given. It is proved that both kinds of implications coincide. Some additional properties are studied like contrapositive symmetry, the exchange principle and others. In particular, it is proved that contrapositive symmetry holds if and only if $$S$$ is the only Archimedean t-conorm on $$L,$$ and $$T$$ jointly with its $$N$$-dual t-conorm satisfy the Frank equation. Finally, some QL- and NQL-implications are also derived from non-smooth t-norms or non-smooth t-conorms and many examples are given showing that in this non-smooth case, QL- and NQL-implications remain strongly connected.

##### MSC:
 03B52 Fuzzy logic; logic of vagueness 68T37 Reasoning under uncertainty in the context of artificial intelligence
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