Maciej Wygralak Fuzzy sets with triangular norms and their cardinality theory. 1-24 2001 124 Fuzzy Sets and Systems 1 http://www.sciencedirect.com/science/article/B6V05-43XG0C1-1/2/2b15ae16d55b0e86c81590017337cf4a db/journals/fss/fss124.html#Wygralak01