# Relation between the weighted matrix norm and the weights

For a nonsingular matrix $W \in \mathbb{C}^{m\times{}m}$, the weighted vector norm is defined as $||\overrightarrow{x}||_W = ||W\overrightarrow{x}||$. Let $||A||$ denote the induced matrix norm by the original vector norm $||\overrightarrow{x}||$, and $||A||_W$ denote the induced matrix norm by the weighted vector norm $||\overrightarrow{x}||_W$. Prove that if $A \in \mathbb{C}^{m\times{}m}$ then $||A||_W = ||WAW^{-1}||$.

• Hello, R.K. Please see this post about how to ask for help with homework. Also, excellent advice on how to ask a good question in general can be found here. Commented Feb 22, 2014 at 22:14

By definition $$\|A\|_W=\sup_{x\ne 0}\frac{\|WAx\|}{\|Wx\|}=\sup_{x\ne 0}\frac{\|WAW^{-1}(Wx)\|}{\|Wx\|}.$$ But as $W$ is non-singular $$\big\{Wx:x\in\mathbb R^n\smallsetminus\{0\}\big\}=\big\{y:y\in\mathbb R^n\smallsetminus\{0\}\big\},$$ and hence $$\sup_{x\ne 0}\frac{\|WAW^{-1}(Wx)\|}{\|Wx\|}=\sup_{y\ne 0}\frac{\|WAW^{-1}y\|}{\|y\|}=\|WAW^{-1}\|.$$