$\{1,2,3,4,5\}$
{ 1 , 2 , 3 , 4 , 5 } \{1,2,3,4,5\} {1,2,3,4,5}
$f(i)=
\left\{\begin{matrix}
1,i\in Q \\
-1,i\notin Q
\end{matrix}\right.$
f ( i ) = { 1 , i ∈ Q − 1 , i ∉ Q f(i)=\left\{\begin{matrix}1,i\in Q\\-1,i\notin Q \end{matrix}\right. f(i)={1,i∈Q−1,i∈/Q
