On two types of discrete implications
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 agree. 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. (c) 2005 Elsevier Inc. All rights reserved.
t-norm t-conorm strong negation finite chain ql-implication nql-implication smoothness implication operator label