Pierre Pradic (LIP, ÉNS Lyon), A realizability notion for MSO over ω



Church’s synthesis problem asks whether there exists a finite-state stream transducer satisfying a given input-output specification. For specifications written in Monadic Second-Order Logic over infinite words, Church’s synthesis can theoretically be solved algorithmically using automata and games, at the price of a non-elementary complexity. We revisit Church’s synthesis via the Curry-Howard correspondence by introducing SMSO, a non-classical subsystem of MSO, which is shown to be sound and complete w.r.t. synthesis thanks to a realizability model inspired by Colin's fibration of automatas over infinite trees. Extracting stream transducers from SMSO proofs is still non-elementary from an algorithmic point of view due to the rule of bounded comprehension.

Joint work with Colin Riba