Matrix Properties / Features

3. Diagonals. Being a grid of rows and columns, a matrix consists of multiple diagonals, but one diagonal has a special significance: the main diagonal.
   
   The main diagonal of the matrix $A_{n \times m} = [a_{i,j}]$ is the array of all elements of the form: $a_{i,i}$, where $1 \le i \le \min(n, m)$. That is,
   $\text{diag}(A) = [a_{1,1}, a_{2,2}, a_{3,3}, \dots, a_{\min(n, m), \min(n, m)}]$.
   Examples:
   
   

   Matrix	$\begin{bmatrix} \boldsymbol{\color{green}{1}} & 2 & 3 \\ 4 & \boldsymbol{\color{green}{5}} & 6 \end{bmatrix}$	$\begin{bmatrix} \color{green}{\bf \text{Bla}} \end{bmatrix}$	$\begin{bmatrix} \boldsymbol{\color{green}{a}} & b \\ c & \boldsymbol{\color{green}{d}} \\ e & f \\ g & h \\ i & j \end{bmatrix}$	$\begin{bmatrix} \boldsymbol{\color{green}{65.7}} & 62.4 & 73.8 & 76.6 & 75.0 \end{bmatrix}$	$\begin{bmatrix} \boldsymbol{\color{green}{1}} & 0 & 0 & 0 \\ 0 & \boldsymbol{\color{green}{1}} & 0 & 0 \\ 0 & 0 & \boldsymbol{\color{green}{1}} & 0 \\ 0 & 0 & 0 & \boldsymbol{\color{green}{1}} \end{bmatrix}$
   Name	$A_{2 \times 3}$	$S_{1 \times 1}$	$L_{5 \times 2}$	$T_{1 \times 5}$	$I_{4 \times 4} = I_4$
   Main Diag	$\text{diag}(A) = [1, 5]$	$\text{diag}(S) = [\text{Bla}]$	$\text{diag}(L) = [a, d]$	$\text{diag}(T) = [65.7]$	$\text{diag}(I) = [1, 1, 1, 1]$