On Synchronized Multitape and Multihead Automata

摘要

Motivated by applications to verification problems in string manipulating programs, we look at the problem of whether the heads in a multitape automaton are synchronized. Given an n-tape pushdown automaton M with a one-way read-only head per tape and a right end marker $ on each tape, and an integer k ≥ 0, we say that M is k-synchronized if at any time during any computation of M on any input n-tuple (x_1,...,x_2) (whether or not it is accepted), no pair of input heads that are not on $ are more than k cells apart. This requirement is automatically satisfied if one of the heads has reached $. Note that an n-tuple (x_1,. .. ,x_n) is accepted if M reaches the configuration where all n heads are on $ and M is in an accepting state. The automaton can be deterministic (DPDA) or nondeterministic (NPDA) and, in the special case, may not have a pushdown stack (DFA, NFA). We obtain decidability and undecidability results for these devices for both one-way and two-way versions. We also consider the notion of k-synchronized oneway and two-way multihead automata and investigate similar problems.