[Tex/LaTex] Why is there a larger vertical space after the first subequation

equationssubequationsvertical alignment

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}
%
\begin{equation}
\label{eq:omega0}
\Omega_0 
\end{equation}
%
\begin{equation}
\label{eq:omega1}
\Omega_1 
\end{equation}
%
\begin{equation}
\label{eq:omega2}
\Omega_2 
\end{equation}
%
\begin{equation}
\label{eq:omega3}
\Omega_3 
\end{equation}
%
\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?

Best Answer

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 \begin{equation} ... \end{equation}, 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}

output of example code

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.

Related Question