# alignat inside align environment?

Why can't I nest an alignat environment in an align environment?

\begin{equation}
\label{eq:115}
\left\{
\begin{aligned}
\begin{alignat*}{2}
&U(x,y,z,t)=\int\frac{x'-x}{r^3}[& &(x'-x)u(x',y',z',t)\\
&                                &+&(y'-y)v(x',y',z',t)\\
&                                &+&(z'-z)w(x',y',z',t)]d\omega',
\end{alignat*}\\
\end{aligned}
\right.
\end{equation}


The following compiles, but the first "+" sign of the second equation line doesn't align up very well with the phantom "+" of the first equation:

\begin{equation}
\begin{dcases}
\label{eq:115}
{\begin{aligned}
U(x,y,z,t)=\int\frac{x'-x}{r^3}[&\phantom{+}(x'-x)u(x',y',z',t)\\
&+(y'-y)v(x',y',z',t)\\
&+(z'-z)w(x',y',z',t)]d\omega',
\end{aligned}}\\
\end{dcases}
\end{equation}


thanks

the easiest fix is to use your second attempt, but instead of just \phantom{+}, surround the plus sign by empty groups to force the correct spacing:

\begin{equation}
\begin{dcases}
\label{eq:115}
{\begin{aligned}
U(x,y,z,t)=\int\frac{x'-x}{r^3}[&\phantom{{}+{}}(x'-x)u(x',y',z',t)\\
&+(y'-y)v(x',y',z',t)\\
&+(z'-z)w(x',y',z',t)]d\omega',
\end{aligned}}\\
\end{dcases}
\end{equation}


for your first attempt, alignat* isn't meant to be embedded. instead, alignedat (defined by mathtools) is appropriate. but you will still need to add empty groups around plus signs, in this case, the two that are between the ampersands:

\begin{equation}
\label{eq:115}
\left\{
\begin{aligned}
\begin{alignedat}{2}
&U(x,y,z,t)=\int\frac{x'-x}{r^3}[& &(x'-x)u(x',y',z',t)\\
&                                &{}+{}&(y'-y)v(x',y',z',t)\\
&                                &{}+{}&(z'-z)w(x',y',z',t)]d\omega',
\end{alignedat}\\
\end{aligned}
\right.
\end{equation} i'm also not sure why you didn't use cases, but that's a niggle.

• Ah, thanks. I didn't know about alignedat*. – Geremia Oct 13 '13 at 2:03

If you want to align parenthesis, this does it.

   \documentclass{article}
\usepackage{mathtools}
\usepackage[T1]{fontenc}
\begin{document}
\begin{equation}
\begin{dcases}
\label{eq:115}
\begin{split}
U(x,y,z,t)=\int\frac{x'-x}{r^3}[\hphantom{{}+{}} &(x'-x)u(x',y',z',t)\\[-2\jot]
{}+{}&(y'-y)v(x',y',z',t)\\
{}+{}&(z'-z)w(x',y',z',t)]d\omega',
\end{split}\\
\end{dcases}
\end{equation}
\end{document} You may prefer to remove \hphantom{{}+{}} if you wish.

• since the + signs continue an existing formula, you should put empty groups around them to get the correct spacing: {}+{}. – barbara beeton Oct 12 '13 at 17:30
• @barbarabeeton Thanks for the official confirmation. – user11232 Oct 12 '13 at 21:44

A few ideas on how to typeset your formulas.

## Code

\documentclass{article}
\usepackage{mathtools} % loads amsmath
\begin{document}

\begin{align}
U(x,y,z,t) & = \int\frac{x'-x}{r^3} (x',y',z',t)
\bigl[ (x'-x)u + (y'-y)v + (z'-z)w \bigr] d\omega' \\
V(x,y,z,t) & = \ldots \\
W(x,y,z,t) & = \ldots
\end{align}

\begin{equation}\left\{\begin{aligned}
U(x,y,z,t) & = \int\frac{x'-x}{r^3} (x',y',z',t)
\bigl[ (x'-x)u + (y'-y)v + (z'-z)w \bigr] d\omega' \\
V(x,y,z,t) & = \ldots \\
W(x,y,z,t) & = \ldots
\end{aligned}\right.\end{equation}

\begin{equation}\left\{\begin{gathered}
\begin{multlined}
U(x,y,z,t) = \int\frac{x'-x}{r^3}  \\
\bigl[ (x'-x)u(x',y',z',t) + (y'-y)v(x',y',z',t) + (z'-z)w(x',y',z',t) \bigr] \\
d\omega'
\end{multlined} \\
V(x,y,z,t) = \ldots \hfill\\
W(x,y,z,t) = \ldots \hfill
\end{gathered}\right.\end{equation}
\end{document}


## Output • I do want the 2nd and 3rd equations on one line, though. – Geremia Oct 13 '13 at 2:04