Comments to ‘logics of public communications’

This is a commentary on Jan Plaza's 'Logics of Public Communications', reprinted in this same issue. Take your average publication on the dynamics of knowledge. In one of its first paragraphs you will probably encounter a phrase like "a logic of public announcements was first proposed by Plaza in 1989 (Plaza, 1989)." Tracking down this publication seems easy, because googling its title 'Logics of Public Communications' takes you straight to Jan Plaza's website where it is online available in the author's own version, including, on that page, very helpful and full bibliographic references to the proceedings in which it originally appeared. Those proceedings are then somewhat harder to find. In fact, I have never seen them. Unfortunately, for the research community, Plaza's work has never been followed up by a journal version. I am very grateful to the editor Wiebe van der Hoek of the journal 'Knowledge, Rationality, and Action' to correct this omission. Plaza's work is reprinted as such, without an update encompass- ing more than fifteen additional years of research in this area. This commentary aims to provide some background to bridge that gap. Public announcement In modal logics of knowledge or belief, infor- mation change can be expressed with dynamic modal operators. It is customary to introduce those modal operators in their 2-version, or 'necessity'-version, such that () stands for "after every execution of program , formula (holds). Instead of 'program', is sometimes called event, action, update, ... The diamond version of that operation is then introduced by notational abbreviation as hi which is by defi- nition equivalent to ¬()¬ . Public announcements are a specific form of the epistemic programsmentioned above: instead of () we now have (') , which stands for "after (every) announcement of ', ." Its diamond form is therefore h 'i , which stands for "there is an execution of 'announce ' truthfully', and after that is true," in other words "' is true and after announcing ', is true." We are now very close to how Plaza models public announcement. Plaza's public announcement What Plaza calls 'public communica- tion' is now commonly named 'public announcement'. Plaza's primitive c 2007 Kluwer Academic Publishers. Printed in the Netherlands.