Positive Announcements

Hans van Ditmarsch, Tim French, James Hales

Research output: Contribution to journalArticlepeer-review

12 Citations (Scopus)


Arbitrary public announcement logic (APAL) reasons about how the knowledge of a set of agents changes after true public announcements and after arbitrary announcements of true epistemic formulas. We consider a variant of arbitrary public announcement logic called positive arbitrary public announcement logic (APAL+), which restricts arbitrary public announcements to announcement of positive formulas. Positive formulas prohibit statements about the ignorance of agents. The positive formulas correspond to the universal fragment in first-order logic. As two successive announcements of positive formulas need not correspond to the announcement of a positive formula, APAL+ is rather different from APAL. We show that APAL+ is more expressive than public announcement logic PAL, and that APAL+ is incomparable with APAL. We also provide a sound and complete infinitary axiomatisation.

Original languageEnglish
Pages (from-to)639-681
Number of pages43
JournalStudia Logica
Issue number3
Early online date7 Oct 2020
Publication statusPublished - Jun 2021


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