区間 (数学)
[ a , b ] := { ( x 1 , ... , x n ) ∈ R n ∣ a 1 ≤ x 1 ≤ b 1 , ... , a n ≤ x n ≤ b n } . {\displaystyle [a,b]:=\{(x_{1},\dots ,x_{n})\in \mathbb {R} ^{n}\mid a_{1}\leq x_{1}\leq b_{1},\dots ,a_{n}\leq x_{n}\leq b_{n}\}.}