Two currently active strands of research on logics for multi-agent systems are dynamic epistemic logic, focusing on the epistemic consequences of actions, and logics of coalitional ability, focusing on what coalitions of agents can achieve by cooperating strategically. In this paper we bridge these topics by considering the question: "what can a coalition achieve by making public announcements?". We propose an extension of public announcement logic with constructs of the form G φ, where G is a group of agents, with the intuitive meaning that G can jointly execute a publicly observable action such that φ will be true afterwards. Actions here are taken to be truthful public announcements, but turn out also to include sequences of such joint actions as well as protocols with alternating actions by different agents, in response to the actions of others. We also study in detail the difference between 'knowing how' (knowing de re) and 'knowing that' (knowing de dicto) in our framework: both can elegantly be expressed in the single-agent case. We present several meta-logical properties of this Group Announcement Logic, including a sound and complete axiomatization, expressivity and the complexity of model checking.

In Alternating-time Temporal Logic (atl), one can express statements about the strategic ability of an agent (or a coalition of agents) to achieve a goal φ such as: "agent i can choose a strategy such that, if i follows this strategy then, no matter what other agents do, φ will always be true". However, strategies in atl are revocable in the sense that in the evaluation of the goal φ the agent i is no longer restricted by the strategy she has chosen in order to reach the state where the goal is evaluated. In this paper we consider alternative variants of atl where strategies, on the contrary, are irrevocable. The difference between revocable and irrevocable strategies shows up when we consider the ability to achieve a goal which, again, involves (nested) strategic ability. Furthermore, unlike in the standard semantics of atl, memory plays an essential role in the semantics based on irrevocable strategies.

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Abstract.We study Normative Temporal Logic (ntl), a formalism intended for reasoning about the temporal properties of normative systems. ntl is a generalisation of the well-known branching-time temporal logic ctl, in which the path quantifiers A ("on all paths. . . ") and E ("on some path. . . ") are replaced by the indexed deontic operators Oη ("it is obligatory in the context of the normative system η that . . . ") and Pη ("it is permissible in the context of the normative system η that. . . "). After introducing the logic, we give a sound and complete axiomatisation. We then present a symbolic representation language for normative systems, and we identify four different model checking problems, corresponding to whether or not a model is represented symbolically or explicitly, and whether or not we are given a concrete interpretation for the normative systems named in formulae to be model checked. We show that the complexity of model checking varies from p-complete in the simplest case (explicit state model checking where we are given a specific interpretation for all normative systems in the formula) up to exptime-hard in the worst case (symbolic model checking, no interpretation given). We present examples to illustrate the use of ntl, and conclude with discussions of related work (in particular, the relationship of ntl to other deontic logics), and some issues for future work.

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