Partially Ordered Two-way B'uchi Automata

We introduce partially ordered two-way B"uchi automata and characterize their expressive power in terms of fragments of first-order logic FO[<]. Partially ordered two-way B"uchi automata are B"uchi automata which can change the direction in which the input is processed with the constraint that whenever a state is left, it is never re-entered again. Nondeterministic partially ordered two-way B"uchi automata coincide with the first-order fragment Sigma2. Our main contribution is that deterministic partially ordered two-way B"uchi automata are expressively complete for the first-order fragment Delta2. As an intermediate step, we show that deterministic partially ordered two-way B"uchi automata are effectively closed under Boolean operations. A small model property yields coNP-completeness of the emptiness problem and the inclusion problem for deterministic partially ordered two-way B"uchi automata.