Language Recognition by Marking Automata

An off-line, memory-restricted Turing machine model, the marking automaton (MA) is presented here as a device strictly intermediate between finite and linear bounded automata. Although much more restricted than the latter, MA are shown capable of recognizing, deterministically, various kinds of context-free (CF) languages and an important related class, as well as such non-CF languages as {xx}. It is not known whether all CF languages are recognizable by MA; however, among the familiar subclasses shown to consist of MA recognizable languages are the Dyck, standard, and bounded CF languages. More importantly, each member of the class of structured CF languages, consisting of all structural descriptions (Phrase-markers) of the sentences in a CF language, is shown to be MA recognizable. The closure of the MA recognizable languages under various set (e.g., boolean) operations is revealed in the proof that all bounded CF languages are MA recognizable.