Van-Nam Huynh Tu Bao Ho Yoshiteru Nakamori An Overview on the Approximation Quality Based on Rough-Fuzzy Hybrids. 493-515 2008 Fuzzy Sets and Their Extensions: Representation, Aggregation and Models http://dx.doi.org/10.1007/978-3-540-73723-0_25 series/sfsc/2008-220 db/series/sfsc/sfsc220.html#HuynhHN08