Krzysztof Dyczkowski Singularity and fuzziness of fuzzy sets. 697-701 2003 EUSFLAT Conf. http://www.eusflat.org/publications/proceedings/EUSFLAT_2003/papers/2Dyczkowski.pdf conf/eusflat/2003 db/conf/eusflat/eusflat2003.html#Dyczkowski03