Inquisitive Neighborhood Logic

We explore an inquisitive modal logic designed to reason about neighborhood models. This logic is based on an inquisitive strict conditional operator <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>⇛</mml:mo></mml:math> , which quantifies over neighborhoods, and which can be applied to both statements and questions. In terms of this operator we also define two unary modalities <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>⊞</mml:mo></mml:math> and , which act respectively as a universal and existential quantifier over neighborhoods. We prove that the expressive power of this logic matches the natural notion of bisimilarity in neighborhood models. We show that certain fragments of the language are invariant under certain modifications of the set of neighborhoods, and use this to show that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>⇛</mml:mo></mml:math> is not definable from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>⊞</mml:mo></mml:math> and , and that questions embedded under <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>⇛</mml:mo></mml:math> are indispensable. We provide a sound and complete axiomatization of our logic, both in general and in restriction to some salient frame classes, establish decidability via the finite model property, and discuss the relations between our logic and other modal logics interpreted over neighborhood models.