Let $X$ and $Y$ be normed vector spaces over the same scalar field. Suppose $X\hookrightarrow Y$ is a continuous embedding, and let $j:X\to Y$ denote the inclusion map. Then
paragraph
admin
\begin{align*}
\|j\|_{\mathcal L(X,Y)}=\inf\{C\ge 0:\|x\|_Y\le C\|x\|_X\text{ for every }x\in X\}.
\end{align*}