timmurphy.org

There is no therefore symbol in the default LaTeX packages. There are (at least) two ways of getting the symbol:

The package amssymb contains the \therefore symbol definition. To use, do something like:

\usepackage{amssymb}
...
$\therefore 1 \neq 2$

Alternatively, you can create your own therefore definition:

\def\therefore{
\leavevmode
\lower0.1ex\hbox{$\bullet$}
\kern-0.2em\raise0.7ex\hbox{$\bullet$}
\kern-0.2em\lower0.2ex\hbox{$\bullet$}
\thinspace}

The amssymb therefore has tiny dots, whereas the definition above has large dots. Take your pick.