How to Synchronize the Heads of a Multitape Automaton

Given an n-tape automaton M with a one-way read-only head per tape and a right end marker $ on each tape, we say that M is aligned or 0-synchronized (or simply, synchronized) if for every n-tuple x=(x1, …, xn) that is accepted, there is a computation on x such that at any time during the computation, all heads, except those that have reached the end marker, are on the same position. When a head reaches the marker, it can no longer move. As usual, an n-tuple x=(x1, …, xn) is accepted if M eventually reaches the configuration where all n heads are on $ in an accepting state. In two recent papers, we looked at the problem of deciding, given an n-tape automaton of a given type, whether there exists an equivalent synchronized n-tape automaton of the same type. In this paper, we exhibit various classes of multitape automata which can(not) be converted to equivalent synchronized multitape automata.