A Two-Way Automaton with Fewer States than Any Equivalent One-Way Automaton
Bruce H. Barnes · IEEE Transactions on Computers · 1971
This correspondence presents an example of a two-way automaton which has significantly fewer states than any one-way automaton accepting the same set of tapes. Thus, in this particular case, memory space can be saved by using a two-way automaton. This savings in space, however, is accompanied by an increase in recognition time.