\documentclass{article}

\begin{document}
A simple equation \verb|\frac{2 a}{3 b}|
$$
\frac{2a}{3b}
$$
A simple equation with additional space \verb|\frac{2 a} {3 b}|
$$
\frac{2 a}{3 b}
$$

\section [Section without space]{A section title without space}

\section [Section with space] {A section title with space}

\end{document}

