Blackburn, Patrick; de Rijke, Maarten; Venema, Yde (2001). Modal Logic. Cambridge Tracts in Theoretical Computer Science. ISBN 9780521527149. 9780521527149 ↩
van Benthem, Johan (2010). Modal Logic for Open Minds (PDF). CSLI. S2CID 62162288. Archived from the original (PDF) on 2020-02-19. https://web.archive.org/web/20200219165057/https://pdfs.semanticscholar.org/9bea/866c143326aeb700c20165a933f583b16a46.pdf ↩