How to place brace on the right of a subset of aligned equations?

Question

This question is essentially an extension of this question about how to place right braces around cases.

I want to have three equations, all aligned, but I want only the latter two of the equations to be grouped by a right brace. I've tried things like

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{aligned}
        \delta\psi&=i\beta\gamma^{5}\
        .
        \delta\sigma&=2\beta\pi\
        \delta\pi&=-2\beta\sigma
        \right\rbrace
\end{aligned}

which obviously doesn't work, as the linked \left\. ... \right\rbrace can't work across the line break in the aligned environment.

Essentially, I want something like this

but the right brace only connecting the lower two equations.

Any help would be much appreciated!

Uhm, I'm sure this has already popped out but right now I can find only this. — campa, Nov 08 '21 at 11:50
Check the first example in amsmath's user guide, sec. 3.7 "Alignment building blocks". — muzimuzhi Z, Nov 08 '21 at 11:51
math mode - Enumerate over mathmode items with partial curly brace - TeX - LaTeX Stack Exchange — user202729, Nov 08 '21 at 12:00
Oh, here is what I was looking for. Does Side brace only some equations help? — campa, Nov 08 '21 at 12:01

egreg · Accepted Answer · 2021-11-10T10:14:31.903

You can use nicematrix:

\documentclass{article}
\usepackage{amsmath}
\usepackage{nicematrix}
\begin{document}
[
\setlength{\arraycolsep}{0pt}
\renewcommand{\arraystretch}{1.2}
\begin{NiceArray}{r >{{}}c<{{}} l}
  \delta\psi   &=&  i\beta\gamma^{5} \
  \delta\sigma &=&  2\beta\pi \
  \delta\pi    &=& -2\beta\sigma
\CodeAfter\SubMatrix.{2-1}{3-3}}
\end{NiceArray}
]
\end{document}

A couple LaTeX runs may be needed in order for the result to be final.

In this particular case, I find it more appealing to use left alignment for the first column: if you change the column specifications into

\begin{NiceArray}{l >{{}}c<{{}} l}

you get

What about text beside the brace? Here we need to better control the intercolumn spaces.

\documentclass{article}
\usepackage{amsmath}
\usepackage{nicematrix}
\begin{document}
[
\renewcommand{\arraystretch}{1.2}
\begin{NiceArray}{ @{} l @{} >{{}}c<{{}} @{} l l @{} }
  \delta\psi   &=&  i\beta\gamma^{5} \
  \delta\sigma &=&  2\beta\pi & \Block{2-1}{\text{whatever}}\
  \delta\pi    &=& -2\beta\sigma
\CodeAfter\SubMatrix.{2-1}{3-3}}
\end{NiceArray}
]
\end{document}

Thank you! This is great, really neat, and exactly what I'm looking for. Just to add - say I wanted to add text to the right of the brace. How would I go about that? I'm not sure I fully understand the syntax of the nicematrix package — LC400, Nov 10 '21 at 10:05

score 2 · Answer 2 · answered Nov 08 '21 at 12:15

You can hide the first row with a negative space (and compensate with a positive space elsewhere)

\documentclass{article}
\usepackage{amsmath}
\begin{document}
align\vspace{\baselineskip}
[
\left.
\begin{aligned}
        \noalign{\vspace{-\baselineskip}}
        \delta\psi&=i\beta\gamma^{5}\
        \delta\sigma&=2\beta\pi\
        \delta\pi&=-2\beta\sigma
\end{aligned}
\right\rbrace  x
]
\end{document}

How to place brace on the right of a subset of aligned equations?

2 Answers2

Linked