Let V {\displaystyle {}V} denote a vector space over K {\displaystyle {}{\mathbb {K} }} , endowed with an inner product ⟨ − , − ⟩ {\displaystyle {}\left\langle -,-\right\rangle } . Let ‖ − ‖ {\displaystyle {}\Vert {-}\Vert } denote the associated norm.
K = R {\displaystyle {}{\mathbb {K} }=\mathbb {R} } , the relation
holds, and, in case K = C {\displaystyle {}{\mathbb {K} }=\mathbb {C} } , the relation