# Enlarge space between horizontal line and equation

How can I enlarge the space between the horizontal line and the equation

\begin{table}[H]
\centering
\begin{center}
\begin{tabular}{|c|} \hline  \begin{aligned} \sum_n \frac{1}{a+n^2}=A(a)\\ \end{aligned}  \\ [0.1ex] \hline \end{tabular}
\end{center}
\end{table}


There are several things that you should remove.

1. table is useless here;
2. center adds unwanted vertical space;
3. and more.

The equation environment already centers its contents and leaves some vertical space. You don't need more and \boxed is your friend here.

\documentclass{article}
\usepackage{amsmath}

\usepackage{lipsum} % for context

\begin{document}

\lipsum[1][1-3]
$$\boxed{ \sum_n \frac{1}{a+n^2}=A(a) }$$
\lipsum[2][1-3]

\end{document}


You might want to make your own version of \boxed where you can specify a different padding.

\documentclass{article}
\usepackage{amsmath}

\usepackage{lipsum} % for context

\newcommand{\Boxed}[2][\fboxsep]{%
\begingroup
\setlength{\fboxsep}{#1}%
\boxed{#2}%
\endgroup
}

\begin{document}

\lipsum[1][1-3]
$$\Boxed{ \sum_n \frac{1}{a+n^2}=A(a) }$$
\lipsum[2][1-3]
$$\Boxed[6pt]{ \sum_n \frac{1}{a+n^2}=A(a) }$$
\lipsum[3][1-3]

\end{document}