We give two generic proofs for cut elimination in propositional modal logics, interpreted over coalgebras. We first investigate semantic coherence conditions between the axiomatisation of a particular logic and its coalgebraic semantics that guarantee that the cut-rule is admissible in the ensuing sequent calculus. We then independently isolate a purely syntactic property of the set of modal rules that guarantees cut elimination. Apart from the fact that cut elimination holds, our main result is that the syntactic and semantic assumptions are equivalent in case the logic is amenable to coalgebraic semantics. As applications we present a new proof of the (already known) interpolation property for coalition logic and newly establish the interpolation property for the conditional logics CK and CK + ID.

The generic modal reasoner CoLoSS covers a wide variety of logics ranging from graded and probabilistic modal logic to coalition logic and conditional logics, being based on a broadly applicable coalgebraic semantics and an ensuing general treatment of modal sequent and tableau calculi. Here, we present research into optimisation of the reasoning strategies employed in CoLoSS. Specifically, we discuss strategies of memoisation and dynamic programming that are based on the observation that short sequents play a central role in many of the logics under study. These optimisations seem to be particularly useful for the case of conditional logics, for some of which dynamic programming even improves the theoretical complexity of the algorithm. These strategies have been implemented in CoLoSS; we give a detailed comparison of the different heuristics, observing that in the targeted domain of conditional logics, a substantial speed-up can be achieved.

Abstract. Motivated by the fact that nearly all conditional logics are axiomatised by so-called shallow axioms (axioms with modal nesting depth ≤ 1) we investigate sequent calculi and cut elimination for modal logics of this type. We first provide a generic translation of shallow axioms to (one-sided, unlabelled) sequent rules. The resulting system is complete if we admit pseudo-analytic cut, i.e. cuts on modalised propositional combinations of subformulas, leading to a generic (but sub-optimal) decision procedure. In a next step, we show that, for finite sets of axioms, only a small number of cuts is needed between any two applications of modal rules. More precisely, completeness still holds if we restrict to cuts that form a tree of logarithmic height between any two modal rules. In other words, we obtain a small (PSPACE-computable) representation of an extended rule set for which cut elimination holds. In particular, this entails PSPACE decidability of the underlying logic if contraction is also admissible. This leads to (tight) PSPACE bounds for various conditional logics. 1

We discuss recent work generalising the basic hybrid logic with the difference modality to any reasonable notion of transition. This applies equally to both subrelational transitions such as monotone neighbourhood frames or selection function models as well as those with more structure such as Markov chains and alternating temporal frames. We provide a generic canonical cut-free sequent system and a terminating proof-search strategy for the fragment without the difference modality but including the global modality. Keywords: Global Modality, Difference Modality, Coalgebraic Semantics, Cut-free Sequent System