Oblivious two-way finite automata: Decidability and complexity

We investigate the descriptional complexity and decidability of obliviousness for two-way finite automata. In particular, we consider the simulation of two-way deterministic finite automata (2DFAs) by oblivious 2DFAs, the simulation of oblivious 2DFAs by sweeping 2DFAs and one-way nondeterministic finite automata (1NFAs) as well as the simulation of sweeping 2DFAs by 1NFAs. In all cases exponential lower bounds on the number of states are obtained for languages over an alphabet with at most four letters. Finally, a procedure for deciding obliviousness of an arbitrary 2DFA is given and, moreover, the problem is shown to be PSPACE-complete.