# Are the definitions of recursively enumerate equivalent?

There are a couple of definitions of recursively enumerable, for example in Judah: $$A \subset \mathbb{N}$$ is called r.e. if there exist a $$\Sigma^0_1$$ formula $$\varphi(x)$$ such that

$$A:=\{n \in \mathbb{N}: \mathbb{N} \vDash \varphi(n)\}$$

In Shoenfield: A predicate $$P$$ is r.e. if there is a recursive predicate $$Q$$ such that

$$P(\bar{a}) \leftrightarrow \exists x Q(\bar{a}, x)$$ for all $$\bar{a} \in P$$. And in here there is another one A new definition of recursively enumerable set?.

My question is: Are the definitions of recursively enumerate equivalent? Why?

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## 1 Answer

Your two definitions are essentially the same: a $$\Sigma_1^0$$ formula is one of the form $$\exists x Q(n,x),$$ where $$Q$$ is computable and $$n$$ is the parameter.

Another difference between the two definitions is that the first only defines r.e. sets, i.e. unary relations, whereas the second defines r.e. relations of arbitrary arity. The two definitions coincide in the case of unary relations.

• In the first definition why is $Q$ computable (recursaive)? I only can be sure $Q$ is Bounded quantifier. – Tom Ryddle Oct 5 '18 at 4:33
• This is the definition of $\Sigma_1^0$. – Yuval Filmus Oct 5 '18 at 4:34
• Thank you Yuval. Can you suggest me some basic books about recursively enumerable and recursive sets? – Tom Ryddle Oct 5 '18 at 4:51
• It's a standard topic so there should be many textbooks out there. Sipser probably covers this topic in his Introduction to the Theory of Computation, for example. – Yuval Filmus Oct 5 '18 at 4:59