For w = 0 {\displaystyle {}w=0} , the statement holds. So suppose w ≠ 0 {\displaystyle {}w\neq 0} , hence, also ‖ w ‖ ≠ 0 {\displaystyle {}\Vert {w}\Vert \neq 0} holds. Therefore, we have the estimates
Multiplication with ‖ w ‖ 2 {\displaystyle {}\Vert {w}\Vert ^{2}} and taking the square root yields the result.