For some linear map $A\in \mathscr{L}(\mathbb{R}^{n};\mathbb{R}^{m})$, the expression $$\lVert A \rVert _{\mathbb{R}^{n},\mathbb{R}^{m}}=\sup\{ \lVert A(\boldsymbol x) \rVert _{\mathbb{R}^{m}}\mid \lVert \boldsymbol x \rVert _{\mathbb{R}^{n}}=1 \}$$is the operator norm of $A$.