Philippe Balbiani Petar Iliev Dimiter Vakarelov A Modal Logic for Pawlak's Approximation Spaces with Rough Cardinality n. 451-464 2008 83 Fundam. Inform. 4 http://iospress.metapress.com/content/k1444200m317u923/ db/journals/fuin/fuin83.html#BalbianiIV08