Two-Way Counter Machines and Diophantine Equations

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Abstract
Let Q be the class of deterministic two-way one-counter machines accepting only bounded languages. Each machine in Q has the property that in every accepting computation, the counter makes at most a fixed number of reversals. We show that the emptiness problem for Q is decidable. When the counter is unrestricted or when the machine is provided with two reversal-bounded counters, the emptiness problem becomes undecidable. The decidability of the emptiness problem for Q is useful in proving the solvability of some numbertheoretic problems. It can also be used to prove that the language L = {u1iu2i2|i≥0} cannot be accepted by any machine in Q (u1 and u2 are distinct symbols). The proof technique is new in that it does not employ the usual "pumping", "counting", or "diagonal" argument. Note that L can be accepted by a deterministic two-way machine with two counters, each of which makes exactly one reversal.

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