R69-19 On the Elimination of Endmarkers

This paper is written by automata theorists for automata theorists. In numerous definitions of automata, there is an "input head" which is expected to be positioned at one square of an "input tape" at all times. There is often the possibility that the automaton might cause the input head to move right, say, from the rightmost input square, before theautomaton has had a chance to do all the computation it wanted to do. Thus, "endmarkers"—special symbols that appear on the leftmost and rightmost input squares but are not otherwise considered part of the input-are often part of the automaton definition. This paper demonstrates that in several cases the endmarkers are unnecessary in that a class of automata with end markers recognizes only languages that are recognizable by the analogous classes of automata without one or both of the endmarkers.