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I don't think it should because a third stack would be superfluous. The machine could just reuse the first stack after it uses the second right?

I'm just beginning to learn about Turing machines, so any help appreciated!

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The idea would be to prove that anything you can do with a 3-stack machine can be simulated by a 2-stack machine. – gt6989b Oct 15 '12 at 1:03
A two-stack automaton is equivalent in power to a Turing Machine; adding a third stack won't give you any more power. – Rick Decker Oct 15 '12 at 13:04

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