On the Difference Between One and Many (Preliminary Version)

We examine the following question: ‘Given a problem, is it more difficult to tell how many solutions the problem has than just deciding whether it has a solution?’. We show, that in specific cases, the question can be put into a mathematically meaningful form, namely when we can translate ‘number of solutions’ as ‘number of distinct accepting computations of a nondeterministic Turing machine’ (perhaps with appropriate weights). In this context, as we show, these questions are equivalent to problems about probabilistic machines (in the sense of Gill (9)).

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