Wiebe van der Hoek Jan Jaspars Elias Thijsse Persistence and Minimality in Epistemic Logic. 62-76 http://link.springer.de/link/service/series/0558/bibs/1489/14890062.htm 1998 conf/jelia/1998 JELIA db/conf/jelia/jelia98.html#HoekJT98