Special Matrices

2. A singleton matrix matrix consists only of one element.
   • This is a square matrix of size $1 \times 1$
   • We denote a singleton matrix $A$ as $A_{1 \times 1} =$$\; A_1 =$$\; [a_{1,1}] =$$\; [a]$ (

     A_{1 \times 1} = A_1 = [a_{1,1}] = [a]

     ).
     • Note that a singleton matrix that contains a number isn't a number: this is a still a matrix. That is, if $A = [a]$ is singleton, then $A \not\in \mathbb{R}$; instead, $a \in \mathbb{R}$.
   
   Examples:

   3. $S_1 = \begin{bmatrix} \text{Bla} \end{bmatrix}$

   18. $I_1 = \begin{bmatrix} 1 \end{bmatrix}$

   19. $Z_1 = \begin{bmatrix} 0 \end{bmatrix}$

   20. $T_1 = \begin{bmatrix} 90.3 \end{bmatrix}$

   21. $E_1 = \begin{bmatrix} 🗽 \end{bmatrix}$