2. Set $A$ satisfies: if $a \in A$, then $\frac{1}{1-a} \in A$. If $2 \in A$, then the set $A$ with the minimum number of elements that satisfies the condition is
(2009, Shanghai Jiao Tong University Independent Recruitment Exam)
Answer: $\left\{2,-1, \frac{1}{2}\right\}$.