Spacing problem between equation and surrounding text

Question

I want to insert formula in text and I used equation as shown:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

So, $\rho : G \rightarrow GL(n,\mathbb{F})$ is representation if and only if

\begin{equation*}
\forall g,h \in G) \hspace{2mm} \rho(gh)=\rho(g)\rho(h).
\end{equation*} 

\end{document}

My problem is too big vertical spacing before and after formula.

Answer 1

If you truly believe that the default values of the length parameters \abovedisplayskip and \belowdisplayskip make LaTeX insert a bit too much vertical whitespace above and below a displayed equation, you could reduce those values a bit. E.g.,

\addtolength\abovedisplayskip{-1ex}
\addtolength\belowdisplayskip{-1ex}

Don't go overboard, though.

Do also follow barbara beeton's advice not to leave blank lines above and below the code that defines the displayed equations. And, if your document loads the setspace package and uses a spacing setting other than \singlespacing, do load the package with the option nodisplayskipstretch. Doing so turns off line stretching before and after displays. If your document uses \doublespacing you'll notice a huge effect.

Answer 2

You can use \useshortskip from the nccmath package — but will change only \abovedisplayskip – or a centerenvironment — the difference is very small – or create a myequ environment, that adds smallskip to \baselineskip (this may be changed, of course).

Here is a code that shows these different possibilities:

\documentclass{article}
\usepackage{amsmath, amsfonts}
\usepackage{nccmath}
\usepackage{etoolbox}
\newenvironment{myequ}{%
\smallskip\par\centering$\displaystyle}
{$\smallskip\par}

\begin{document}

\noindent With the \verb+equation*+ environment : \medskip

So, $\rho : G \rightarrow GL(n,\mathbb{F})$ is representation if and only if
\begin{equation*}
\forall g,h \in G \hspace{2mm} \rho(gh)=\rho(g)\rho(h).
\end{equation*}
Text text text text text text text text text text text text text text text text text text text text. \\

\noindent With a \verb+center + environment : \medskip

So, $\rho : G \rightarrow GL(n,\mathbb{F})$ is representation if and only if
\begin{center}
$\displaystyle
 \forall g,h \in G \hspace{2mm} \rho(gh)=\rho(g)\rho(h). $
\end{center}
Text text text text text text text text text text text text text text text text text text text text. \\

\noindent With \verb+\useshortskip+ : \medskip

So, $\rho : G \rightarrow GL(n,\mathbb{F})$ is representation if and only if\useshortskip
\begin{equation*}
\forall g,h \in G \hspace{2mm} \rho(gh)=\rho(g)\rho(h).
\end{equation*}
Text text text text text text text text text text text text text text text text text text text text. \\

\noindent With the \verb+myequ+ environment : \medskip

So, $\rho : G \rightarrow GL(n,\mathbb{F})$ is representation if and only if\useshortskip
\begin{myequ}
\forall g,h \in G \hspace{2mm} \rho(gh)=\rho(g)\rho(h).
\end{myequ}
Text text text text text text text text text text text text text text text text text text text. \\

\end{document} 

Answer 3

In addition to Barbara's comment about removing the blank line, you can manually set line spacing using the spacing environment from the setspace package (see this link! from the LaTeX wikibook).

Working example:

\documentclass{article}
\usepackage{amsmath}
\usepackage{setspace}

\begin{document}

\begin{spacing}{0.2}
So, $\rho : G \rightarrow GL(n,\mathbb{F})$ is representation if and only if
\begin{equation*}
    \forall g,h \in G) \hspace{2mm} \rho(gh)=\rho(g)\rho(h).
\end{equation*} 
\end{spacing}

\end{document}