Align Equations Including `cases`

Question

I want a series of aligned equations, the last of which has two cases. See the figure for an example.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
    a &= b \\
&\begin{cases}
    \ge c \\
    \le d
\end{cases}
\end{align*}
\end{document}

Naturally, I want the =/≥/≤ signs to be aligned.

This issue crops up quite often in my work. I always have to come up with some rather hacky, non-generalisable way using \mathllap{\Bigg\{}. But, it's rather unsatisfying to hack it each time—and the hack won't work on many scenarios—so I wonder if there is a better, more direct way?

Answer 1

You could use the nicematrix package as shown in https://tex.stackexchange.com/a/583373/36296

\documentclass{article}
\usepackage{nicematrix}
\begin{document}

\[
\begin{NiceArray}{r@{\;}l}[first-row]
a&= b \\ 
&\ge c\\
&\le d
\CodeAfter \SubMatrix{\{}{1-2}{2-2}{.}[left-xshift=0.1em]
\end{NiceArray}
\]

\end{document}

Answer 2

Adaptations

• insert a negative space before the cases environment: \hspace{-.73em}
• use relative size em, so it works ok (not perfectly, as there is a small difference for huge) also for other fontsizes
• I defined an environment alignedcases for this. This way it is easier to type and you could change it later on, if you find a better solution.

\newenvironment{alignedcases}{
    \hspace{-.73em}
    \begin{cases}
}{
    \end{cases}
}

Result

Code

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{tikzmark}

\newenvironment{alignedcases}{
    \hspace{-.73em}
    \begin{cases}
}{
    \end{cases}
}

\newcommand{\test}[1]{
    \begin{align*}
        a &\tikzmark{#1}= b \\
        &\begin{alignedcases}
            \ge c \\
            \le d
        \end{alignedcases}
    \end{align*}
    %
    \begin{tikzpicture}[overlay, remember picture]
        \draw [red] ([xshift=.31em, yshift=.75em] {pic cs:#1}) -- ++(0,-4.5em);
    \end{tikzpicture}
}


\begin{document}

\footnotesize\test{footnotesize}
\small\test{small}
\normalsize\test{normalsize}
\large\test{large}
\LARGE\test{LARGE}
\huge\test{huge}

\end{document}

Answer 3

It's tricky:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
    \begin{align*}
        a &= b \\
        &\hspace*{-.2cm}\begin{cases}
            \ge c \\
            \le d
        \end{cases}
    \end{align*}
\end{document}

... but works:

Answer 4

This may be overkill, but you could use the tikzmark library to add a calligraphic brace to the align* environment, without using cases at all. One advantage is that you don't need to measure anything or approximate negative spacing. This also allows for the easy addition of text to the brace.

Note that you need to add {} to the tikzmarknode for proper spacing.

\documentclass{article}
\usepackage{tikz, amsmath}
\usetikzlibrary{tikzmark, decorations.pathreplacing, calligraphy}
\tikzset{mybrace/.style={decorate, decoration={calligraphic brace, amplitude=1.5mm}, thick}}

\begin{document}

\begin{align*}
a&=b\\
&\tikzmarknode{A}{{}\ge c}\\
&\tikzmarknode{B}{{}\le d}
\end{align*}

\begin{tikzpicture}[overlay, remember picture]
\draw[mybrace] (B.south west)--node[left=1mm]{some text}(A.north west);
\end{tikzpicture}

\end{document}

Answer 5

I have received some excellent answers—and extremely quickly! None are quite as satisfying as I had hoped. Sometimes, there are some seemingly-simple things that LaTeX struggles with, but hey!

Given that there was no simple solution, I include below another version which I had used. As with most of the other answers, it's a bit of a hack. However, it does get the =/≥/≤ signs perfectly aligned; just, the left brace isn't necessarily quite the right size—and \left\{ doesn't work here.

\documentclass{article}
%\usepackage{amsmath}
\usepackage{mathtools}
\begin{document}
\begin{align*}
    a &= b \\
\hspace*{+0.25em}
\mathllap{\Bigg\{}
\hspace*{-0.25em}
&\begin{aligned}
    &\ge c \\
    &\le d
\end{aligned}
\end{align*}
\end{document}