# What is a dual norm, and how to compute it?

I came across a concept called dual norm in my optimization course, which I am not familiar with. I am trying to understand what it is and how to compute it.

from Wiki, the definition of the dual norm is,

Let $${\displaystyle \|\cdot \|}$$ be a norm on $${\mathbb {R} ^{n}.}$$ The associated dual norm, denoted $$\|\cdot\|_{*}$$ is defined as $$\|z\|_{*}=\sup \left\{z^{\top} x \mid\|x\| \leq 1\right\}$$

My questions are:

1. What is this dual norm used for, why do we need it?

2. Is that the primal norm can be any like 1-norm, 2-norm, inf-norm, and the dual norm is still defined as above?

3. How to compute this dual norm? Does Python or Matlab provide such a function? If not, I noticed that it is essentially an optimization problem, with an objective function: maximize:$$z^{\top} x$$, and an inequality constraint $$\|x\| \leq 1$$, can I compute it in this way?

• math.stackexchange.com/questions/265721/… Feb 13, 2021 at 15:57
• Yes the definition of the dual is the same for any norm and actually the dual of the 1-norm is the inf-norm (and vice-versa). You have correctly identified that computing the dual norm can be done by solving a (convex) optimization problem.
– Surb
Feb 13, 2021 at 18:22