John T. Rickard Janet Aisbett Greg Gibbon Fuzzy Subsethood for Fuzzy Sets of Type-2 and Generalized Type- n. 50-60 2009 17 IEEE T. Fuzzy Systems 1 http://dx.doi.org/10.1109/TFUZZ.2008.2006369 db/journals/tfs/tfs17.html#RickardAG09