Fuzzy Sets and Systems : Special Issue Celebrating the 50th Anniversary of Fuzzy Sets (2015)

International audienceThis special issue celebrates the publication of the first paper on fuzzy sets by its founder Lotfi Zadeh in 1965.1 Without any doubt, this paper is the most important landmark for the research field in general and for Fuzzy Sets and Systems in particular. In fact, the mere existence of the oldest scientific journal on the topic is only due to Zadeh's publication. On this occasion, we invited a number of esteemed colleagues, many of whom are serving the journal as members of the editorial board, to deliver their thoughts on their topic of predilection and competence. We thus managed to collect twenty position papers that altogether provide a relatively exhaustive overview of the current activity on fuzzy sets and their applications, and of questions regarding the role and appropriateness of fuzzy sets in various topics. Thus, going beyond a pure celebration of the past, this issue is also meant to indicate promising directions of future research on fuzzy sets. A tout seigneur tout honneur,2 we are extremely lucky to publish one more paper by the author of the publication we celebrate. Lotfi Zadeh himself provides his own testimony about the genesis of his first paper on fuzzy sets, and he also expresses his opinions concerning new directions to be explored in the future. In a second contribution, Dubois and Prade provide a glimpse at the various areas of research that were influenced by fuzzy sets, pointing out mature fields, barren directions, and promising areas of research. One of their main points is the necessity of interpreting membership functions in applied research. It is important to articulate the contribution of fuzzy sets in the various problems they are applied to. This question is crucial to explain where degrees of membership come from and how they can be measured. The answer will be different if membership functions encode ideas of similarity, of uncertainty or yet of preference. The rest of the issue is clustered into various themes: logic, mathematics, systems engineering, decision and optimization, data analysis and management