  Any kind of dynamics in dynamic epistemic logic can be represented as an
action model. Right? Wrong! In this contribution we prove that the update
expressivity of communication patterns is incomparable to that of action
models. Action models, as update mechanisms, were proposed by Baltag, Moss, and
Solecki in 1998 and have remained the nearly universally accepted update
mechanism in dynamic epistemic logics since then. Alternatives, such as arrow
updates that were proposed by Kooi and Renne in 2011, have update equivalent
action models. More recently, the picture is shifting. Communication patterns
are update mechanisms originally proposed in some form or other by Agotnes and
Wang in 2017 (as resolving distributed knowledge), by Baltag and Smets in 2020
(as reading events), and by Velazquez, Castaneda, and Rosenblueth in 2021 (as
communication patterns). All these logics have the same expressivity as the
base logic of distributed knowledge. However, their update expressivity, the
relation between pointed epistemic models induced by such an update, was
conjectured to be different from that of action model logic. Indeed, we show
that action model logic and communication pattern logic are incomparable in
update expressivity. We also show that, given a history-based semantics and
when restricted to (static) interpreted systems, action model logic is
(strictly) more update expressive than communication pattern logic. Our results
are relevant for distributed computing wherein oblivious models involve
arbitrary iteration of communication patterns.
