Abstract
We study the computational complexity of important decision problems — including general membership, fixed-machine membership, emptiness, disjointness, equivalence, containment, universe, and finiteness problems — for various restrictions and combinations of two-way nondeterministic reversal-bounded multicounter machines (2NCM) and two-way pushdown automata. We show that the general membership problem (respectively fixed membership problem) for 2NCM is NP-complete (respectively in P).We then give applications to some problems in coding theory. We examine generalizations of various types of codes with marginal errors. For example, a language L is k-infix-free if there is no non-empty string y in L that is an infix of more than k strings in L−{y}. Our general results imply the complexity of determining whether a given machine accepts a k-infix-free language, for one- and two-way deterministic and nondeterministic finite automata (answering an open question from the literature).
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