HyperLTL Satisfiability is Σ_1^1-complete, HyperCTL* Satisfiability is Σ_1^2-complete
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Temporal logics for the specification of information-flow properties are able to express relations between multiple executions of a system. The two most important such logics are HyperLTL and HyperCTL*, which generalise LTL and CTL* by trace quantification. It is known that this expressiveness comes at a price, i.e. satisfiability is undecidable for both logics. In this paper we settle the exact complexity of these problems, showing that both are in fact highly undecidable: we prove that HyperLTL satisfiability is Σ_1^1-complete and HyperCTL* satisfiability is Σ_1^2-complete. These are significant increases over the previously known lower bounds and the first upper bounds. To prove Σ_1^2-membership for HyperCTL*, we prove that every satisfiable HyperCTL* sentence has a model that is equinumerous to the continuum, the first upper bound of this kind. We prove this bound to be tight. Finally, we show that the membership problem for every level of the HyperLTL quantifier alternation hierarchy is Π_1^1-complete.
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