Agreement theorems in dynamic-epistemic logic

In this paper we bring Aumann's Agreement Theorem to dynamic-epistemic logic. We show that common belief of posteriors is sucient for agreements in \epistemic-plausibility models", under common and well-founded priors, from which the usual form of agreement results, using common knowledge, follows. We do not restrict to the nite case, and show that in countable structures such results hold if and only if the underlying \plausibility ordering" is well-founded. We look at these results from a syntactic point of view, showing that neither well-foundedness nor common priors are expressible in a commonly used language, but that the static agreement result is nitely derivable in an extended modal logic. We nally consider \dynamic" agreement results, and show they have a counterpart in epistemic-plausibility models. We also show to which agreements one gets via \public announcements." A comparison of the two types of dynamic agreements reveals that they can indeed be dierent. nite case, thus improving on known qualitative agreement theorems (2), and show that in countable structures such results hold if and only if the underlying \plausibility ordering" is well-founded. We then look at these results from a syntactic point of view, showing that neither well-foundedness nor common priors are expressible in the language proposed in (3), even extended with a common belief operator, but we also show a nitary syntactic derivation of the static agreement result in an extended modal language. We