mtprosymbols
I'm currently using Math Time Pro2, but considering the \forall and \exists is not suitable for my liking. How can I change back to the origin LaTeX \forall and \exists symbol?
One more question, is that is there a more-straight (double) (left) right arrow? (I mean, in the "head" of the arrow) The \rightarrow-like symbol all have curly embellishment, which I don't want.
This is the most modest increment on Seamus' and Martin Scharrer's comments above: if you use it more than once, you may as well go ahead and define
\newcommand\bigforall{\mbox{\Large $\mathsurround0pt\forall$}}
which will co-operate with the mathmode context you will likely be using the symbol in.
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}
Best Answer
You can load a math symbol font:
Look at the comparison and decide whether you really want the change.