## Subsection1.2.5Unit ball

In 3-dimensional space, the notion of the unit ball is intuitive: the set of all points that are a (Euclidean) distance of one from the origin. Vectors have no position and can have more than three components. Still the unit ball for the 2-norm is a straight forward extension to the set of all vectors with length (2-norm) one. More generally, the unit ball for any norm can be defined:

###### Definition1.2.5.1. Unit ball.

Given norm $\| \cdot \| : \Cm \rightarrow \mathbb R \text{,}$ the unit ball with respect to $\| \cdot \|$ is the set $\{ x ~\vert ~ \| x \| = 1 \}$ (the set of all vectors with norm equal to one). We will use $\| x \| = 1$ as shorthand for $\{ x ~\vert ~ \| x \| = 1 \} \text{.}$

###### Homework1.2.5.1.

Although vectors have no position, it is convenient to visualize a vector $x \in \R^2$ by the point in the plane to which it extends when rooted at the origin. For example, the vector $x = \left( \begin{array}{c} 2 \\ 1 \end{array} \right)$ can be so visualized with the point $( 2, 1 ) \text{.}$ With this in mind, match the pictures on the right corresponding to the sets on the left:

(a) $\| x \|_2 = 1 \text{.}$

(1)

(b) $\| x \|_1 = 1 \text{.}$

(2)

(c) $\| x \|_\infty = 1 \text{.}$

(3)

Solution

(a) $\| x \|_2 = 1 \text{.}$

(3)

(b) $\| x \|_1 = 1 \text{.}$

(1)

(c) $\| x \|_\infty = 1 \text{.}$

(2)