Andrzej Skowron Jaroslaw Stepaniuk James F. Peters Approximation of Information Granule Sets. 65-72 2000 conf/rsctc/2000 Rough Sets and Current Trends in Computing http://link.springer.de/link/service/series/0558/bibs/2005/20050065.htm db/conf/rsctc/rsctc2000.html#SkowronSP00