The inequality ( ∑ i x i y i ) 2 ≤ ( ∑ i x i 2 ) ( ∑ i y i 2 ) {\displaystyle \textstyle (\sum _{i}x_{i}y_{i})^{2}\leq (\sum _{i}x_{i}^{2})(\sum _{i}y_{i}^{2})} or its generalization |⟨x,y⟩| ≤ ||x|| ||y||.
