MSO+nabla is undecidable
arXiv:1901.06900
Abstract
This paper is about an extension of monadic second-order logic over the full binary tree, which has a quantifier saying ``almost surely a branch π \in {0, 1}^w satisfies a formula ϕ(π)''. This logic was introduced by Michalewski and Mio; we call it MSO+nabla following notation of Shelah and Lehmann. The logic MSO+nabla subsumes many qualitative probabilistic formalisms, including qualitative probabilistic CTL, probabilistic LTL, or parity tree automata with probabilistic acceptance conditions. We show that it is undecidable to check if a given sentence of MSO+nabla is true in the full binary tree.
Accepted to LICS 2019