How to type three points diagonally?

Question

In an equation, I want to display three points like "...", but instead of horizontally, they should be diagonally. I'm sure there must be a command to do that, but a google search did not really help.

Any idea?

Answer 1

The command is called \ddots. See also How to look up a symbol or identify a math symbol or character? (esp. detexify).

Answer 2

According to https://texfaq.org/FAQ-mathlips

The mathdots package (besides fixing up the behaviour of (La)TeX \ddots and \vdots when the font size changes) provides an “inverse diagonal” ellipsis \iddots

That is, \iddots is three dots sloping forwards while \ddots is three dots sloping backwards.

Answer 3

An alternative to the \iddots (inverse diagonal dots) from the mathdots package:

\makeatletter
\def\Ddots{\mathinner{\mkern1mu\raise\p@
\vbox{\kern7\p@\hbox{.}}\mkern2mu
\raise4\p@\hbox{.}\mkern2mu\raise7\p@\hbox{.}\mkern1mu}}
\makeatother

Then call with \Ddots

Answer 4

It's not perfect but you can use \dots and \cdot and align them using subscript _ and superscript ^ as demonstrated in row 2 below.

You get the vertical dots on row 1 as a bonus since the need to write diagonal dots are typically related to building a "dotted" matrix like in the picture below.

\newcommand{\vertdots}{\underset{\big{\overset{\cdot}{\cdot}}}{\cdot}} 
\newcommand{\diagdots}{_{^{\big\cdot}\cdot _{\big\cdot}}}

\begin{equation*}
A = \begin{bmatrix} 
a           & \dots         & b         \\ 
\vertdots   &  \diagdots    & \vertdots \\
c           & \dots         & d         \\
\end{bmatrix}
\end{equation*}

This produces a matrix that looks like this:

I tried to align the diagonal dots better but didn't succeed. This will be good enough for me.