How to have local alignment groups within the same 'align' environment?

Question

For the following, I need to find a cleaner way of having one alignat environment and aligning the first two lines with each other while aligning the last two lines with each other so that the vertical spacing is consistent across all the lines.

\documentclass{article}
\usepackage{mathtools, nccmath}
\begin{document}
    \begin{fleqn}[0pt]
        \begin{alignat}{2}
            & a &&= b\
            %
            & x &&= y
        \end{alignat}\vspace*{-2\baselineskip}
        \begin{alignat}{2}
            & a &&= b\
            %
            & xxx &&= yyy
        \end{alignat}
    \end{fleqn}
\end{document}

Are you interested in the equation numbering at all? – Werner Nov 25 '23 at 18:43 — Werner, Nov 25 '23 at 18:43

score 4 · Accepted Answer · answered Nov 25 '23 at 17:14

You can remove \belowdisplayskip and \abovedisplayskip, but also take care that the interline spacing in displays is enlarged by \jot.

\documentclass[twocolumn]{article}
\usepackage{mathtools, nccmath}
\begin{document}
\begin{fleqn}[14em]
\begin{alignat}{2}
  & a &&= b\
  & x &&= y
\end{alignat}\par\nopagebreak
\vspace*{\dimeval{-\abovedisplayskip-\belowdisplayskip+\jot}}\noindent
\begin{alignat}{2}
  & a &&= b\
  & xxx &&= yyy
\end{alignat}
\end{fleqn}
\end{document}

Note. I used twocolumn and 14em just in order to move the equations nearer to the numbers and better show the alignment in a smaller picture.

Heiko Theißen · Answer 2 · 2023-11-25T18:37:39.427

This solution (which is inspired by code from appendix D of The TeXbook) follows each alignat group with a statement that works its way recursively up the vertical list and re-inserts only the \hboxes and \penaltys produced by the alignment while removing vertical space from the bottom and top.

\documentclass{article}
\usepackage{mathtools, nccmath}
\def\unpack{\unskip\unpenalty\unskip % remove space from bottom
  \count0=\lastpenalty\unpenalty
  \setbox0=\lastbox
  \ifhbox0{\unpack}\box0 \penalty\count0 \else
  \unskip\unskip\unpenalty\unskip\unskip\unskip % remove space from top
  \fi}
\begin{document}
\begin{fleqn}[0pt]
\vbox{
\begin{alignat}{2}
  & a &&= b\\
  & x &&= y
\end{alignat}
\par\unpack
\begin{alignat}{2}
  & a &&= b\\
  & xxx &&= yyy
\end{alignat}
\par\unpack
}
\end{fleqn}
\end{document}

The \vbox is necessary because \lastbox cannot be used on the main vertical list.

See my edited answer. – Heiko Theißen Nov 25 '23 at 18:38 — Heiko Theißen, Nov 25 '23 at 18:38

How to have local alignment groups within the same 'align' environment?

2 Answers2