relation-symbolssymbols
The not equal symbol (≠), has its slash from the upper-right to the bottom-left. For example:
\begin{equation}
i \neq j
\end{equation}
Typesets the following:
i ≠ j
How can I obtain a similar symbol, whose slash directionality is reversed (i.e., from the upper-left to the bottom-right, as displayed below)?
I recommend using a vertically centered colon which matches the equal sign better, such as \vcentcolon from the mathtools package. You can also change it generally to vertical by \mathtoolsset{centercolon}.
Compare standard :=
to the vertically centered:
So your definition could become
\usepackage{mathtools}
\newcommand{\defeq}{\vcentcolon=}
\newcommand{\eqdef}{=\vcentcolon}
with tighter spacing. And since both symbols are already relation symbols, as egreg said, we even could omit \mathrel.
And you could do that even easier by mathtools provided commands:
\documentclass{article}
\usepackage{mathtools}
\begin{document}
$a\coloneqq b \qquad b\eqqcolon a$
\end{document}
You need the amssymb package in order to use the \leqslant and \geqslant symbols.
\documentclass{article}
\usepackage{amssymb}
\begin{document}
\[ a-b\geqslant 0 \iff a \geqslant b \iff b \leqslant a.\]
\end{document}
Best Answer
You could make use of
\reflectbox(as has been pointed out in the comments), but it won't be usable in super- or subscripts: