Adam Bjorndahl

Topological Subset Space Models for Public Announcements

We reformulate a key definition given by Wang and Agotnes [18] to provide semantics for public announcements in subset spaces. More precisely, we interpret the precondition for a public announcement of \(\varphi \) to be the “local truth” of \(\varphi \), semantically rendered via an interior operator. This is closely related to the notion of \(\varphi \) being “knowable”. We argue that these revised semantics improve on the original and offer several motivating examples to this effect. A key insight that emerges is the crucial role of topological structure in this setting. Finally, we provide a simple axiomatization of the resulting logic and prove completeness.

Reasoning about justified belief

Halpern and Pass [8] introduce a logic of justified belief and go on to prove that strong rationalizability is characterized in this logic in terms of common justified belief of rationality (CJBR). Their paper provides semantics for this logic but no axiomatization. We correct this deficiency by reformulating the definition of justified belief and providing a complete axiomatization of this new system. We then prove a result analogous to the characterization of strong rationalizability in terms of CJBR, and analyze the additional assumptions needed to do so.

Reasoning about rationality

We provide a sound and complete axiomatization for a class of logics appropriate for reasoning about the rationality of players in games, and show that essentially the same axiomatization applies to a very wide class of decision rules. We also consider games in which players may be uncertain as to what decision rules their opponents are using, and define in this context a new solution concept, DD-rationalizability.

The Epistemology of Nondeterminism

This paper proposes new semantics for nondeterministic program execution, replacing the standard relational semantics for propositional dynamic logic (PDL). Under these new semantics, program execution is represented as fundamentally deterministic (i.e., functional), while nondeterminism emerges as an epistemic relationship between the agent and the system: intuitively, the nondeterministic outcomes of a given process are precisely those that cannot be ruled out in advance. We formalize these notions using topology and the framework of dynamic topological logic (DTL). We show that DTL can be used to interpret the language of PDL in a manner that captures the intuition above, and moreover that continuous functions in this setting correspond exactly to deterministic processes. We also prove that certain axiomatizations of PDL remain sound and complete with respect to the corresponding classes of dynamic topological models. Finally, we extend the framework to incorporate knowledge using the machinery of subset space logic, and show that the topological interpretation of public announcements coincides exactly with a natural interpretation of test programs.

Bayesian Games with Intentions

We show that standard Bayesian games cannot represent the full spectrum of belief-dependent preferences. However, by introducing a fundamental distinction between intended and actual strategies, we remove this limitation. We define Bayesian games with intentions, generalizing both Bayesian games and psychological games, and prove that Nash equilibria in psychological games correspond to a special class of equilibria as defined in our setting.