Treffer: Backtracking games and inflationary fixed points

We define a new class of games, called backtracking games. Backtracking games are essentially parity games with an additional rule allowing players, under certain conditions, to return to an earlier position in the play and revise a choice or to force a countback of the number of moves. This new feature makes backtracking games more powerful than parity games. As a consequence, winning strategies become more complex objects and computationally harder. The corresponding increase in expressiveness allows us to use backtracking games as model-checking games for inflationary fixed-point logics such as IFP or MIC. We identify a natural subclass of backtracking games, the simple games, and show that these are the right model-checking games for IFP by (a) giving a translation of formulae φ and structures U into simple games such that U =φ if, and only if, Player 0 wins the corresponding game and (b) showing that the winner of simple backtracking games can again be defined in IFP.