Norm

A norm on a vector space $V$ over $\mathbb{F}$ is a function $\|\cdot\|:V\to[0,\infty)$ such that for all $u,v\in V$ and $a\in\mathbb{F}$:

Here $|a|$ denotes the absolute value of the scalar $a$ (for $\mathbb{F}=\mathbb{R}$ or $\mathbb{C}$).

A norm induces a metric by $d(u,v)=\|u-v\|$, making $V$ into a metric space and thus giving notions of convergence and continuity.

Examples:

• On $\mathbb{R}^n$, the Euclidean norm $\|x\|_2=\sqrt{\sum_{i=1}^n x_i^2}$ is a norm.
• On $\mathbb{R}^n$, the $\ell^1$ norm $\|x\|_1=\sum_{i=1}^n |x_i|$ and the max norm $\|x\|_\infty=\max_i |x_i|$ are norms.
• On continuous functions on $[0,1]$, the sup norm $\|f\|_\infty=\sup_{t\in[0,1]} |f(t)|$ is a norm.