This is my code.
\documentclass{article}
\usepackage{amssymb,amsmath,amsthm,enumitem}
\begin{document}
How can I decrease vertical space between text above and below \$\$ \ldots \$\$ ?
$$x^{2}+y^{2} = 1$$
How can I decrease vertical space between text above and below \$\$ \ldots \$\$ ?
And how can I decrease vertical space between text above and below align command ?
\begin{align*}
abdefg &= 1\\
&\leq 2.
\end{align*}
How can I decrease vertical space between text above and below align command ?
\end{document}
And it shows as this picture

Everyone know how to fix it? Please help.

\lipsum[1]from thelipsumpackage, for example) and see if the vertical space is smaller. – Sigur Jan 26 '15 at 11:36\[ \]instead. – Sigur Jan 26 '15 at 11:37