# Singular Value Decomposition - Transpose and Dot Product

matrix

In this third post of the series, we will introduce two additional building blocks.

## Transpose ¶

The transpose of a column vector $\mathbf{u}$, $\mathbf{u}^\top$, is the row vector of $\mathbf{u}$. The transpose of an $m \times n$ matrix $\mathbf{A}$ is an $n \times m$ matrix whose columns are formed from the corresponding rows of $\mathbf{A}$. For example, if we have

$$ \mathbf{C} = \begin{bmatrix} 5 & 4 & 2 \\ 7 & 1 & 9 \\ \end{bmatrix} $$

then the transpose of $\mathbf{C}$ is:

$$ {\mathbf{C}} = \begin{bmatrix} 5 & 7 \\ 4 & 1 \\ 2 & 9 \\ \end{bmatrix} $$

The transpose of a row vector becomes a column vector with the same elements and vice versa. The element in the $i^\text{th}$ row and $j^\text{th}$, $x_\text{ij}$, is equal to the element in the $j^\text{th}$ row and $i^\text{th}$ column, $x_\text{ji}$, of the original matrix. Therefore,

$$ \mathbf{A}^\top_\text{ij} = \mathbf{A}_\text{ji} $$

The transpose has some important properties. First, the transpose of a transpose is itself.

$$ \mathbf{A}^\top_\text{ij} = \mathbf{A}_\text{ji} $$

In addition, the transpose of a product is the product of transpose in the reverse order.

$$ (\mathbf{A}\mathbf{B})^\top = \mathbf{B}^\top\mathbf{A}^\top $$

## Dot product ¶

If we have two vectors $\mathbf{u}$ and $\mathbf{v}$:

$$ \mathbf{u} = \begin{bmatrix} u_1 \\ u_2 \\ \vdots \\ u_n \\ \end{bmatrix} $$

$$ \mathbf{v} = \begin{bmatrix} v_1 \\ v_2 \\ \vdots \\ v_n \\ \end{bmatrix} $$

The dot product (aka inner product) of these vectors is defined as the transpose of $\mathbf{u}$ multipled by $\mathbf{v}$ :

$$ \mathbf{u}\cdot\mathbf{v} = \mathbf{u}^T\mathbf{v} = \begin{bmatrix} u_1 & u_2 & \cdots & u_n \\ \end{bmatrix} \begin{bmatrix} v_1 \\ v_2 \\ \vdots \\ v_n \\ \end{bmatrix} = u_1v_1 + u_2v_2 + \cdots + u_nv_n $$

Based on this definition, the dot product is commutative:

$\mathbf{u}\cdot\mathbf{v} = \mathbf{v}\cdot\mathbf{u}$

In the next post, we will return to the topic of eigenvalues and eigenvectors.