{−3,−1,0,1,3}\{-3,-1,0,1,3\}{−3,−1,0,1,3}
R\mathbb{R} R
R−{−3,−1,0,1,3}\mathbb{R}-\{-3,-1,0,1,3\}R−{−3,−1,0,1,3}
−3−1,0,1,3-3-1,0,1,3−3−1,0,1,3
