# My phantom cup doesn't have the same width as my non-phantom cups in an align environment; how can I fix this?

Here's my code:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align*}
Q= & \phantom{\cup} \{\{n-1,n-1,3\}\} \\
& \cup \{\{n-1,n-j+1,j+1\}: 3 \leq j \leq \lfloor n/2 \rfloor\} \\
& \cup \{\{n-2,n-j+1,j+2\}: 4 \leq j \leq \lfloor n/2 \rfloor-1\}.
\end{align*}

\end{document}


This is what it looks like:

I want the brackets to line up on the left, but it's not behaving. How can I nudge the first bracket to the left to align with the other brackets?

It lines up properly if we don't use a \phantom{\cup}, but I don't actually want the cup there.

• \mathbin{\phantom{\cup}} – LaRiFaRi Aug 7 '15 at 7:16

## 1 Answer

Perhaps this will suit. In the first example, I set the alignment point after the \cup, whereas in the second, I set it after the = sign.

When things like \cup and = are surrounded by math atoms, additional space is provided as separation. Such space is, for example, not present when those glyphs stand alone as in $=$. The problem with \phantom is that it loses track of what surrounds it. Thus, the method of using {}\cup{} is a way to designate using \cup as if something (in this case two empty groups) surrounds it on both sides. In that case, the extra space is inserted.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
Q = \phantom{{}\cup{}}& \{\{n-1,n-1,3\}\} \\
\phantom{=} \cup{}& \{\{n-1,n-j+1,j+1\}: 3 \leq j \leq \lfloor n/2 \rfloor\} \\
\phantom{=} \cup{}& \{\{n-2,n-j+1,j+2\}: 4 \leq j \leq \lfloor n/2 \rfloor-1\}.
\end{align*}
\end{document}


Alternately, this would suffice:

\begin{align*}
Q={} & \phantom{{}\cup{}} \{\{n-1,n-1,3\}\} \\
& \cup \{\{n-1,n-j+1,j+1\}: 3 \leq j \leq \lfloor n/2 \rfloor\} \\
& \cup \{\{n-2,n-j+1,j+2\}: 4 \leq j \leq \lfloor n/2 \rfloor-1\}.
\end{align*}

• For the uninitiated, you may want to explain in words what \phantom{{}\cup{}} achieves that \phantom{\cup} does not. Put differenty, what do the empty groups, {}, that you've placed around \cup "do"? – Mico Aug 7 '15 at 2:15