B3. Two Norm

The two norm of a vector $x \in \mathbb{R}^{n}$, also called Euclidean is defined by

$$\vert\vert \vec{x} \vert\vert _2 = \left ( \sum_{i=1}^{n} \vert x_i\vert^2 \right )^{1/2} .$$ (B4)

The square of the two norm

$$\vert\vert \vec{x} \vert\vert _2^2 = \sum_{i=1}^{n} \vert x_i\vert^2 = \vec{x}^{\cal T} \vec{x}$$ (B5)

is calculated by a simple vector product.