Linear temporal logic (LTL) is a modal logic with the modalities ‘Next Step’, ‘Until’ and ‘Always Going to Be’, and with linear orders as models, which are often called traces. LTL has the expressive power to capture some properties of computation traces, but not all the properties of interest. One class of properties beyond the scope of the logic is characterised by the existence of dependencies between traces. Properties of this kind are referred to as hyperproperties [1]. In this talk we compare the expressive power of two approaches to extending LTL in order to capture hyperproperties. We explore the relation between LTL under team semantics (TeamLTL) and LTL extended with trace quantifiers (HyperLTL), with a focus on the effects of extending the logics with arbitrary disjunctions and negations has on the relative expressivity. In the team semantics setting synchronicity brings with it great expressive power, making the logic incomparable to HyperLTL, whereas without the requirement of synchronicity the logic remains on the level of LTL [2]. Extending the asynchronous logic with connectives familiar from the team semantics literature, namely the Boolean disjunction and Boolean negation, widens what can be characterised in the logic, but how does the relation to HyperLTL fare in these extensions?

We will provide an overview of the results for the synchronous semantics of TeamLTL, while elaborating further on our recent work [3] with the asynchronous semantics of TeamLTL. In that domain we particularly consider the extensions of TeamLTL with the Boolean disjunction and a so-called left-downward closed fragment of the extension of TeamLTL with the Boolean negation, where the negation cannot occur in the left-hand side of the ‘Until’-operator or within the scope of the ‘Always Going to Be’-operator. We show that TeamLTL extended with the Boolean disjunction is equi-expressive with the positive Boolean closure of HyperLTL restricted to one universal quantifier, while the left-downward closed fragment of TeamLTL extended with the Boolean negation is expressively equivalent with the Boolean closure of HyperLTL restricted to one universal quantifier.

This talk is based on joint work with Juha Kontinen and Jonni Virtema.

Bibliography

1. Michael R. Clarkson and Fred B. Schneider, Hyperproperties, Journal of Computer Security, vol. 18 (2010), no. 6, pp. 1157–1210.
2. Andreas Krebs and Arne Meier and Jonni Virtema and Martin Zimmermann, Team Semantics for the Specification and Verification of Hyperproperties, 43rd International Symposium on Mathematical Foundations of Computer Science, MFCS 2018 (Liverpool, UK), (Igor Potapov and Paul G. Spirakis and James Worrell, editors), vol. 117, Schloss Dagstuhl - Leibniz-Zentrum für Informatik, 2018, pp. 10:1–10:16.
3. Juha Kontinen and Max Sandström and Jonni Virtema, Set Semantics for Asynchronous TeamLTL: Expressivity and Complexity, 48th International Symposium on Mathematical Foundations of Computer Science, MFCS 2023 (Bordeaux, France), (Jérôme Leroux and Sylvain Lombardy and David Peleg, editors), vol. 272, Schloss Dagstuhl - Leibniz-Zentrum für Informatik, 2023, pp. 60:1–60:14.