l 2 {\textstyle l^{2}} – это { x = { x n } : ∑ | x n | 2 < ∞ } {\textstyle \{x=\{x_{n}\}:\sum |x_{n}|^{2}<\infty \}}
( x , y ) = ∑ n x n y n {\textstyle (x,y)=\sum _{n}x_{n}y_{n}}
| | x | | = ∑ x n 2 {\textstyle ||x||={\sqrt {\sum x_{n}^{2}}}}