symbols
I want to write a wider \Downarrow, I couldn't find one in any package I know, so I tried with:
\documentclass[11pt,a4paper]{book}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\Big\Downarrow
%\Large\Downarrow
\end{equation}
\end{document}
but it seems that it only gets longer, I want it wider: how can I do that?
Edit: to be clearer I add an image:
on the left: what I have
on the right: what I want.
The \not symbol typesets a zero width character which protrudes on the right and so it is apt to be superimposed only on certain characters having a suitable width (the equal sign, for example); since the two arrows are wider than the equal sign, the alignment is not optimal. A solution is to push the \not a bit on the right and backspacing for the same amount:
\newcommand\nleftrightarrow{
\mathrel{\mkern2.1mu\not\mkern-2.1mu\leftrightarrow}}
\newcommand\nLeftrightarrow{
\mathrel{\mkern2.1mu\not\mkern-2.1mu\Leftrightarrow}}
You might also consider the set of "negated arrows" provided by amssymb; the names are the same that I chose:
\usepackage{amssymb}
$X\nleftrightarrow Y$ and $X\nLeftrightarrow Y$
Try both and choose what suits your taste. If you already load amssymb use \renewcommand instead of \newcommand.
One can also define a \xnot command that gives to the symbol the width of the equal sign and use a standard method for superimposing symbols:
\newcommand\xnot{\mathpalette\xxnot}
\newcommand*\xxnot[2]{\mathrel{\ooalign{%
\hfil$#1\not\mathrel{\hphantom=}$\hfil\cr\hfil$#1#2$\hfil\cr}}}
Now \xnot\leftrightarrow gives the same result as the "definition by hand" before. The strange \mathrel after \not has its explanation in the fact that TeX never adds space between two relation math atoms. Thanks to Martin for having suggested this method. The centernot package does something similar.
Just put two equals signs, backing up slightly:
\newcommand{\eqdef}{\overset{\mathrm{def}}{=\joinrel=}}
This is how TeX builds extensible arrows.
The magic macro \joinrel is defined as
\mathrel{\mkern-3mu}
and the magic is done by the fact that TeX doesn't put spaces between consecutive relation symbols:
=\mathrel{\mkern-3mu}=
will thus result in two equals signs slightly superimposed to each other.
Best Answer
Here is a way to do that while also respecting a correct spacing around the symbol: