Reducing vertical space before and after using align

Question

I am currently having outrageous vertical space before and after I use align.

For a bit of context, I used \onehalfspacing to make my document a bit clearer. However, it becomes ugly when I using align. For instance, here is a picture of what it looks like compared to when I am using array (and in fact any time I need to write maths using "$$") :

I would like to reduce this white space when I use align, but I can't find anything relevant enough. Since I am searching for something that works anytime without always repeating the command each time I have to use align, I was thinking of creating a new environment, but I don't know how to make one like align.

Thanks a lot for your help !

EDIT : Here is the code I used to take the picture :

\documentclass{article}
\usepackage[utf8]{inputenc}

%---------------

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc} 
\usepackage[french]{babel} 
\usepackage[cyr]{aeguill} 

%--------------- 

\usepackage{fancyhdr}
\usepackage[top=3cm, bottom=3cm, left=3cm, right=3cm]{geometry}
\usepackage{setspace}


%---------------

\onehalfspacing 
%\setlength{\parskip}{0cm} %not usefull

%--------------- 

\usepackage{amsmath, amssymb, amsthm} 
\usepackage{mathrsfs}
\usepackage{etoolbox}
\usepackage{stmaryrd}

\title{Tests Overleaf}
\date{\today}

\begin{document}

\maketitle

\section{Introduction}

------------------------------------------------------------------------------------------

I am using array : 

$$\begin{array}{cccccc}
    f : & A & \times & B & \longrightarrow & C \\
    & x & ; & y & \longmapsto & xy 
\end{array}$$

------------------------------------------------------------------------------------------

I am using align* : 

\begin{align*}
    a \times (b+c) & = a \times b + a \times c\\
    & = a \times b + a \times c + 0
\end{align*}

------------------------------------------------------------------------------------------

\end{document}

Answer 1

Never have a blank line before any display math, that forces a one-line paragraph with an empty white line before the display. Also $$ is not supported LaTeX syntax so I changed that to \[.

As the line above the equation is short TeX can use the smaller \abovedisplayshortskip if the equation is to the right of the line end, however align is always full width even if visually narrow, so always uses \abovedisplayskip You can use aligned instead in a way similar to you used array, aligned is more suitable than array as it sets the content in display math mode, array is designed for setting matrices of values, and sets each cell in inline math mode.

\documentclass{article}
\usepackage[utf8]{inputenc}

%---------------

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc} 
\usepackage[french]{babel} 
\usepackage[cyr]{aeguill} 

%--------------- 

\usepackage{fancyhdr}
\usepackage[top=3cm, bottom=3cm, left=3cm, right=3cm]{geometry}
\usepackage{setspace}


%---------------

\onehalfspacing 
%\setlength{\parskip}{0cm} %not usefull

%--------------- 

\usepackage{amsmath, amssymb, amsthm} 
\usepackage{mathrsfs}
\usepackage{etoolbox}
\usepackage{stmaryrd}

\title{Tests Overleaf}
\date{\today}

\begin{document}

\maketitle

\section{Introduction}

------------------------------------------------------------------------------------------

I am using array : 
\[
\begin{array}{cccccc}
    f : & A & \times & B & \longrightarrow & C \\
    & x & ; & y & \longmapsto & xy 
\end{array}
\]

------------------------------------------------------------------------------------------

I am using align* : 
\begin{align*}
    a \times (b+c) & = a \times b + a \times c\\
    & = a \times b + a \times c + 0
\end{align*}

------------------------------------------------------------------------------------------

------------------------------------------------------------------------------------------

I am using aligned* : 
\[
\begin{aligned}
    a \times (b+c) & = a \times b + a \times c\\
    & = a \times b + a \times c + 0
\end{aligned}
\]

------------------------------------------------------------------------------------------

\end{document}

Alternatively the mathtools extension if amsmath provides a command that enables the use of the short display skip on specific displays where you can tell the previous line will not be close to the top line of the alignment.

You may also want to use

\usepackage[nodisplayskipstretch]{setspace}

so setspace does not mess with this spacing: