# Box around equation in align mode?

I've been trying several different approaches to place a box (a highlight red box) around equations in beamer. Though, I usually do not get the expected result, or a bunch of errors pops up and nothing compiles.

Below follows a MWE of three equations inside the align environment (ideally I want to keep this environment). I need a box that can go at each equation at a time (or more than one sometimes, but not all of them at once). Also, some methods I've tried could not place a box around the \sum correctly, usually cutting part of them out, or the iterators below them.

\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\begin{document}
\begin{align}
&  \left[ \omega \sum_{k \in K} y_k + \sum_{i \in V} \sum_{j \in V} \sum_{k \in K} c_{ij}x_{ijk}\right]\\
\notag\\
& \sum_{k \in K} \sum_{j \in V | j \ne i} x_{ijk} = 1 & & \forall i \in P\\
\notag\\
& \sum_{j \in V | j \ne i} x_{ijk} - \sum_{j \in V | j \ne i} x_{dev(i)jk} = 0 && \forall i \in P, k \in K
\end{align}
\end{document}

I'd recommend the hf-tikz package; it requires hand adjustments, but is completely independent of align.

\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{hf-tikz}

\begin{document}

\begin{frame}
\begin{align}
\tikzmarkin{first}(0.2,0.75)(-0.2,-0.7)
& \biggl[
\omega \sum_{k \in K} y_k +
\sum_{i \in V} \sum_{j \in V} \sum_{k \in K} c_{ij}x_{ijk}
\biggr]
\tikzmarkend{first}
\\[2ex]
& \sum_{k \in K} \sum_{j \in V | j \ne i} x_{ijk} = 1
&& \forall i \in P
\\[2ex]
\tikzmarkin{third}(0.2,0.6)(-0.2,-0.75)
& \sum_{j \in V \mid j \ne i} x_{ijk} - \sum_{j \in V \mid j \ne i} x_{dev(i)jk} = 0
&& \forall i \in P, k \in K
\tikzmarkend{third}
\end{align}
\end{frame}

\end{document}

If you just want the red border, with true corners,

\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage[customcolors]{hf-tikz}

\hfsetfillcolor{white}
\hfsetbordercolor{red}

\begin{document}

\begin{frame}
\begin{align}
\tikzmarkin[disable rounded corners=true]{first}(0.2,0.75)(-0.2,-0.7)
& \biggl[
\omega \sum_{k \in K} y_k +
\sum_{i \in V} \sum_{j \in V} \sum_{k \in K} c_{ij}x_{ijk}
\biggr]
\tikzmarkend{first}
\\[2ex]
& \sum_{k \in K} \sum_{j \in V | j \ne i} x_{ijk} = 1
&& \forall i \in P
\\[2ex]
\tikzmarkin[disable rounded corners=true]{third}(0.2,0.6)(-0.2,-0.75)
& \sum_{j \in V \mid j \ne i} x_{ijk} - \sum_{j \in V \mid j \ne i} x_{dev(i)jk} = 0
&& \forall i \in P, k \in K
\tikzmarkend{third}
\end{align}
\end{frame}

\end{document}

• This one seems to work exactly how I wanted it to work. Nice answer, thank you, egreg :) – Lonatico Nov 29 '16 at 22:52

You can obtain it with the \Aboxed command from mathtools, but it can enclose a single &, so I had to remove the alignment of the .

For colour, I patched the \boxed command from amsmath, which is used by \Aboxed (but be aware all math boxed equations will be red). Additionally, I used \smashoperator in case of large subscript, to improve the spacing with the following expression:

\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}}
\usepackage{xpatch}
\xpatchcmd{\boxed}{%
\fbox}{%
\fcolorbox{red}{white}}{}{}

\begin{document}

\begin{align}
\Aboxed{& \Biggl[ \omega\sum_{k \in K} y_k + \sum_{i \in V} \sum_{j \in V} \sum_{k \in K} c_{ij}x_{ijk}\Biggr]}\\
\notag\\
&\sum_{k \in K} \smashoperator[r]{\sum_{j \in V | j \ne i}} x_{ijk}= 1 \qquad \forall i \in P\\
\notag\\
\Aboxed{&\smashoperator[r]{\sum_{j \in V | j \ne i}} x_{ijk} - \smashoperator{\sum_{j \in V | j \ne i}} x_{dev(i)jk}= 0\qquad \forall i \in P, k \in K} \end{align}

\end{document}

• Thank you for the answer, Bernard. I really preferred that the \forall appeared in an extra &. There is no way to do that? Moreover, is there any way to make the box red? – Lonatico Nov 29 '16 at 1:48
• For the rule colour I added a patch to \ boxed. For the other alignment, I don't how to do that: when I tried which & it gobbled the second column. Maybe a work-around using eqparbox? – Bernard Nov 29 '16 at 2:14
• Oh well, I guess I'll have to use this way then. Thanks again anyway, you have been very helpful :) (Edit: I'll wait a bit before voting for best answer, in case anyone appears with a magic answer) – Lonatico Nov 29 '16 at 2:37
• if boxing can't handle &, then the \foralls can be aligned manually, by using \mathrlap of the first part of equation (2) over a phantom of the first part of equation (3), and setting the second part of equation (2) after that.. – barbara beeton Nov 29 '16 at 18:35
• Boxing accepts only one &, it seems. I was thinking of using eqparbox, but didn't have to test yet – Bernard Nov 29 '16 at 20:05