Philippe Balbiani Ilya Shapirovsky Valentin B. Shehtman Every world can see a Sahlqvist world. 69-85 2006 conf/aiml/2006 Advances in Modal Logic http://www.aiml.net/volumes/volume6/Balbiani-Shapirovsky-Shehtman.ps db/conf/aiml/aiml2006.html#BalbianiSS06