# Why is there a larger vertical space after the first subequation?

I am using the subequations environment and observe non-uniform vertical spacing between subequations. Specifically, the space between the first and second equation is larger than the other spaces. Here is the code snippet:

\documentclass[12pt,fleqn,letterpaper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\begin{document}

\begin{subequations}
\label{eq:omegai}
%
$$\label{eq:omega0} \Omega_0$$
%
$$\label{eq:omega1} \Omega_1$$
%
$$\label{eq:omega2} \Omega_2$$
%
$$\label{eq:omega3} \Omega_3$$
%
\end{subequations}

\end{document}


After doing some debugging I found that the offending command appears to be fleqn in the call to \documentclass[12pt,fleqn,letterpaper]{article}. If I remove fleqn the spacing is uniform. Any ideas how to get uniform vertical spacing and still use fleqn?

i really hate to say this, but i think it'a a bug in amsmath. (consider it reported.)

the "official dogma" about grouped formulas is that it's better to use one of the multi-line environments provided by amsmath than to separately enter $$...$$, and in fact, by adopting such an environment, the uneven spacing goes away, at least under some conditions.

here is a possibility that gets rid of the uneven vertical spacing -- but only if the block of subequations is preceded by some text. (but that's likely to be true.)

observe that the gather environment appears only once, and encompasses all the lines in the block, while successive lines are separated by double backslashes. (the amsmath multline environments are meant to be used only with displays that contain more than one line. if applied to single lines, the spacing will always be wrong.)

\documentclass[12pt,fleqn,letterpaper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\begin{document}

\noindent
MMMMMM\hfill MMMMMM%
\begin{subequations}
\label{eq:omegai}
%
\begin{gather}
\label{eq:omega0}
\Omega_0
\\
%
\label{eq:omega1}
\Omega_1
\\
%
\label{eq:omega2}
\Omega_2
\\
%
\label{eq:omega3}
\Omega_3
\end{gather}
%
\end{subequations}
{MMMMMM\hfill MMMMMM\parfillskip=0pt\par}

\end{document}


if only the display block is present in the file, the uneven spacing between the first and second lines may recur, but it's much less likely.

there are quite a few bugs associated with fleqn in amsmath; we dutifully test for them and record them as they are reported. unfortunately, a full review and overhaul hasn't yet been scheduled, and i can't predict when it might happen.

• Thank you for the workaround, changing from equation environments to either align or gather, as you suggested. However, instead of getting a larger space between the first and second equations I get uniformly large spacing between all of the equations (unnecessarily large). This doesn't happen with your MWE, but in my actual application it does: – okj Mar 25 '15 at 21:31
• \documentclass[12pt,fleqn,letterpaper]{article} \usepackage[latin1]{inputenc} \usepackage{amsmath} \usepackage{amsfonts} \usepackage{amssymb} \begin{document} \noindent MMMMMM\hfill MMMMMM% \begin{subequations} \label{eq:designobjectives} % \begin{align} \label{eq:sy11} \max \overline{\sigma_{y1}} \end{align} % \begin{align} \label{eq:s1111} \overline{S_{1111}} = S_{1111}^{sub} \end{align} % \begin{align} \label{eq:d} \min \overline{D} \end{align} % \end{subequations} {MMMMMM\hfill MMMMMM\parfillskip=0pt\par} \end{document} – okj Mar 25 '15 at 21:31
• sorry, not sure how to post latex code in a comment... – okj Mar 25 '15 at 21:33
• @okj -- don't use multiple align environments. that will always insert (unwanted) extra space between lines. just one \begin{align} at the beginning (see how gather is used in the example in the answer), with \\ between the lines, and a single \end{align} at the end. you might want to take a look at the documentation -- texdoc amsmath on a tex live system. – barbara beeton Mar 25 '15 at 21:41
• \noindent even works on its own! – davyjones Aug 17 '17 at 15:48