How to denote diagonal dots symbol in LaTeX?

Latex diagonal dots are the sum of three dots located along the diagonal. This diagonal dots symbol is represented by the \ddots command.

Properties	Value
Symbol	Diagonal dots
Package	Default
Command	\ddots
Argument	No
Example	\ddots →

These diagonal dots are used to represent a matrix with n number of elements.

\documentclass{article}
\begin{document} 
   $$\ddots$$ 
\end{document}

Output :

And you see the diagonal matrix where the elements of the matrix are arranged just along the diagonal. In that case, only the latex diagonal dots symbol is used.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
 \begin{bmatrix}
   a_{1} &  & \\ 
   & \ddots & \\ 
   &  & a_{n}
 \end{bmatrix}
\end{document}

Output :

Inverse diagonal dots

Inverse diagonal dots are located in a diagonal direction from south-west to north-east.

Latex has more than one command and method to represent this inverse diagonal dots symbol.

Package	Command
MySymbol	\udots
Mathdots	\iddots
yhmath	\adots
graphicx	\reflectbox{…}

\documentclass{article}
\usepackage{graphicx}
\usepackage{yhmath}
\usepackage{mathdots}
\usepackage{MnSymbol}
\begin{document}
  $$ \iddots $$
  $$ \udots $$
  $$ \adots $$
  $$ \reflectbox{$\ddots$}$$
\end{document}

Output :

$\ddots$ is passed as an argument in \reflectbox command which converts diagonal dots symbol to inverse diagonal dots symbol. And for this you need to take the help of graphicx package.