Notation in this book

$$\begin{array}{l} i = 1, 2, 3, \cdots, n;\\ j = 1,2,3, \cdots,p;\\ \\ X=\left\{ \begin{matrix} x_{11}&x_{12}&\cdots&x_{1p}\\ x_{21}&x_{22}&\cdots&x_{2p}\\ \vdots&\vdots&\ddots&\vdots\\ x_{n1}&x_{n2}&\cdots&x_{np}\\ \end{matrix} \right\}\\ \\ x_{i}=\left\{ \begin{matrix} x_{i1}\\ x_{i1}\\ \vdots\\ x_{ip}\\ \end{matrix} \right\}\\ \\ x_{j}=\left\{ \begin{matrix} x_{1j}\\ x_{2j}\\ \vdots\\ x_{nj}\\ \end{matrix} \right\} \end{array}$$

$n$

The number of distinct data points, or observations.

$p$

The number of variables that are available for use in making predictions.

$x_{ij}$

The value of the $j$th variable for the $i$th observation

$y_{i}$

The $i$th observation of variable on which we wish to make predictions