Andrzej Skowron Jaroslaw Stepaniuk James F. Peters Roman W. Swiniarski Calculi of Approximation Spaces. 363-378 2006 72 Fundam. Inform. 1-3 http://iospress.metapress.com/openurl.asp?genre=article&issn=0169-2968&volume=72&issue=1&spage=363 db/journals/fuin/fuin72.html#SkowronSPS06