On deterministic context-free languages, multihead automata, and the power of an auxiliary pushdown store

Abstract

A deterministic context-free language L0 is described which is log(n)-complete for the family of languages recognized by deterministic log(n)- tape bounded auxiliary pushdown automata in polynomial time. It follows that L0 is a “hardest” deterministic context-free language (DCFL), since all DCFL's are recognized in polynomial time by deterministic pushdown automata. L0 is, moreover, a simple precedence language and a simple LL(1) language. Thus the tape complexities of these proper subfamilies are essentially the same as the tape complexity of all DCFL's.We show that an auxiliary pushdown store does, in fact, add some power to some restricted families of log(n)-tape bounded Turing machines. The basic result is that every two-way 2k-head nondeterministic finite automation can be replaced by an equivalent two-way k-head nondeterministic pushdown automation. This indicates, also, that every 2k-head nondeterministic finite automation language can be recognized in 0(n3k) steps. Other results relate multihead automata classes with other multihead automata classes, with families recognized by log(n)-tape bounded Turing machines with restricted tape alphabets, and with time-bounded complexity classes.