I have a basic question about the behavior of a potential Turing machine.

Suppose that $S$ is a countable set of binary strings, so that we can enumerate $S$ as $(s_i)_{n\in \mathbb{N}}$.

Suppose that we want to construct a Turing machine $M$ that, on any input, simply writes each element of $S$ on a tape in order $s_1, s_2, \ldots$. (I realize that $M$ will never halt.)

But we can construct such an $M$ if and only if our particular bijection $\mathbb{N} \to S$ is computable, correct? Or is knowing that $S$ is countable somehow enough?


1 Answer 1


By definition, a set $S$ is recursively enumerable if there is a Turing machine that enumerates it.


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