Abstract
It is shown that k + 1 heads are better than k for one-way multihead pushdown (resp. stack) automata if they do not have endmarkers on the input tape and accept by final state with at least one input head at the right end of the input string. In addition, for two-way multihead pushdown (resp. stack) automata, “hardest languages” are described. It is also shown that for two-way multihead pushdown (resp. stack) automata there is a language with the hardest time and space complexity which can be written as L + for some one-way multihead pushdown (resp. stack) automaton language L, where L + = {w|w n is in L for some n ⩾ 1}. A representation theorem for recursively enumerable languages is also given.
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