Solution
= Solution
A subset $A\subseteq\Sigma^*$ is a <computably enumerable set> if there is a program, equivalently a <register machine>, that halts on input $w$ exactly when $w\in A$. It need not halt when $w\notin A$.
= Solution
A subset $A\subseteq\Sigma^*$ is a <computably enumerable set> if there is a program, equivalently a <register machine>, that halts on input $w$ exactly when $w\in A$. It need not halt when $w\notin A$.