Succinctness of Epistemic Languages

Proving that one language is more succinct than another becomes harder when the underlying semantics is stronger. We propose to use Formula-Size Games (as put forward by Adler and Immerman, 2003), games that are played on two sets of models, and that directly link the length of play with the size of the formula. Using FSGs, we prove three succinctness results for m-dimensional modal logic: (1) In system Km, a notion of ‘everybody knows’ makes the resulting language exponentially more succinct for m > 1 (2) In S5m, the same language becomes more succinct for m > 3 and (3) Public Announcement Logic is exponentially more succinct than S5m, if m > 3. The latter settles an open problem raised by Lutz, 2006.