How to disable alignment of individual lines in an align environment?

Question

I often write definitions like

\begin{align*}
   f &: \mathbb{R} \to \mathbb{R}
 \\ f(0) &: 0
 \\ f(x) &= \frac{\sin x}{x}
\end{align*}

^{(Yeah, I know this could also be written with a cases environment, but I'm not asking about that.)}

Needless to say, the alignment is a bit of a mess here. I do want the = signs to be aligned, but it doesn't really make sense for the colon. In principle, that should just be centered by itself, and the equations below centered as well as a block, like would happen with two separate environments

but that has its own disadvantages, including the undesired vertical space.

It's no good to just omit the alignment character

\begin{align*}
   f &: \mathbb{R} \to \mathbb{R}
 \\ f(0) &: 0
 \\ f(x) &= \frac{\sin x}{x}
\end{align*}

as that just smashes the first line entirely in the left alignment column

Nesting environments doesn't do the trick either

\begin{align*}
   f : \mathbb{R} \to \mathbb{R}
 \\
\begin{aligned}
    f(0) &= 0
 \\ f(x) &= \frac{\sin x}{x}
\end{aligned}
\end{align*}

– this seems to result in the same as the first version, for some reason.

What's the proper way?

Related questions:

Alignment like in align environment asks also about a different-aligned first line, but not centered. The solution involves a manual alignment point. That's really awkward and should not be necessary if the first line should just be individually centered.
How to align only some equations in an align environment? is more about adding additional padding to align specific lines within an already aligned environment

FTR, I actually meant to write f(0) = 1, but only noticed the discontinuity at zero in the finished question. Not that it matters for what I'm asking. — leftaroundabout, Jun 08 '23 at 10:25

Mico · Accepted Answer · 2023-06-08T10:57:58.313

You could nest an aligned environment (for rows 2 and 3) inside a gather* environment:

Do also note that I've inserted a typographic strut (via a \vphantom directive) in row 2 in order to separate it a bit more from rows 1 and 3. Furthermore, I would use \colon rather than : in row 1.

\documentclass{article} % or some other suitable document class
\usepackage{amsmath,amssymb}
\begin{document}
\begin{gather}
   f \colon \mathbb{R} \to \mathbb{R} \
   \begin{aligned}
     f(0) &= 1 \vphantom{\frac{\sin x}{x}} \
     f(x) &= \frac{\sin x}{x}
   \end{aligned} 
\end{gather}
\end{document}

How to disable alignment of individual lines in an align environment?

1 Answers1