On Biautomata

We initiate the theory and applications of biautomata. A biautomaton can read a word alternately from the left and from the right. We assign to each regular language L its canonical biautomaton. This structure plays, among all biautomata recognizing the language L, the same role as the minimal deterministic automaton has among all deterministic automata recognizing the language L. We expect that from the graph structure of this automaton one could decide the membership of a given language to certain significant classes of languages. We present the first result of this kind: a language L is piecewise testable if and only if the canonical biautomaton of L is acyclic. From this result the famous Simon’s characterization of piecewise testable languages easily follows.