How to type three points diagonally?

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

According to http://www.tex.ac.uk/cgi-bin/texfaq2html?label=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.

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