| Symbol/Unicode | Diagonal three dots/U+22F1 | 
|---|---|
| Type of symbol | Dots | 
| Package (requirement) | No | 
| Argument | No | 
| Latex command | \ddots | 
| Example | \ddots→ ⋱ | 
Many times we need diagonal three dots in our document. In Latex, you need to use the \ddots command for this.
\documentclass{article}
\usepackage{amsmath}
\begin{document}
   \[
   \begin{bmatrix}
      a_1 & \cdots & \\
      \vdots & \ddots & \\
       & & a_n
   \end{bmatrix}
   \]
\end{document}Output :
