Vertical spacing above and below equation

[gzeus]

I would like to reduce the default vertical spacing before and after
the environment \begin{equation}\end{equation}.
How could I do that ?

[Werner Grundlingh]

You can change the defaults values stored in
\abovedisplayskip and \abovedisplayshortskip;
\belowdisplayskip and \belowdisplayshortskip
These default values are
\abovedisplayskip=12pt plus 3pt minus 9pt
\abovedisplayshortskip=0pt plus 3pt
\belowdisplayskip=12pt plus 3pt minus 9pt
\belowdisplayshortskip=7pt plus 3pt minus 4pt
and handle the vertical skip (or space & glue) between paragraph text
and mathematical equations. The difference between the choice of
\...skip and \...shortskip depends on whether the preceding/following
paragraph line is long/short, and accommodates equations that could
stick out vertically (for example, \sum_{i=1}^k ... in displaystyle).
If you want these values to only be changed locally (i.e., for a
specific length of text or equation only), include it within a group:

\begingroup % or {
\setlength{\abovedisplayskip}{...}
...
\begin{equation} % or \[
...
\end{equation} % or \]
\endgroup % or }

See the example proposed by Herbert Voss in his mathmode.pdf,
available on CTAN from
http://tug.ctan.org/cgi-bin/ctanPackageInformation.py?id=voss-mathmode
More specifically, page 35-36, Section 11.5(.1).

Hope this helps,
Werner