align vs equation: vertical space before equation

Question

I want to remove the vertical spacing before \begin{align*} so it looks like the vertical spacing before \begin{equation*}. To show a screenshot of what I mean:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

hello. equation:

\begin{equation*}
  a + b = c \\
\end{equation*}

hello. align:

% this \vspace will fix the space issue for a single align environment, but I want a solution
% that addresses the issue for all align environments through the entire document.
%\vspace{-\baselineskip}
\begin{align*}
  a + b = c \\
\end{align*}

\end{document}

I can see this issue has been posted before here: Spacing: Align vs. Equation

This I can see the recommendation to use \vspace{-\baselineskip} in front of every single align environment. I want a solution that is done once in the header and is applied consistently throughout the entire document.

I also see the accepted answer involves colored boxes with tcolorbox: I want something simpler.

Also here: align vs equation

I see the recommendation to use:

[fleqn] option and \setlength{\mathindent}{0pt}

This doesn't work when I try it. Secondly, the fleqn option for amsmath is designed to left-align rather than center align environments which isn't what I want to do.

I presume this extra spacing is a full paragraph skip? Why does align do that and equation does not? can I remove that with the align environment?

Answer 1

You can try patching align*, inserting \useshortskip from nccmath at the very beginning, but in my opinion, it is better to use this command manually when necessary:

\documentclass{article}
\usepackage{etoolbox}
\usepackage{amsmath, nccmath}
\AtBeginEnvironment{align*}{\useshortskip}

\begin{document}

hello. equation:
\begin{equation*}
  a + b = c
\end{equation*}

hello. align:
\begin{align*}
  a + b = c
\end{align*}

\end{document}