Align a box in the formula

Question

The following code provides this result:

I would like to get a small "{" fitting the text and not the upper and down box.

Any idea?

\documentclass[preview]{standalone}
 \usepackage{mathtools}
 \usepackage{tikz}
 \setlength{\textwidth}{15cm}
\makeatletter
 \define@key{janbertdims}{align}{\def\janbert@align{\hspace{#1}}}
 \define@key{janbertdims}{arrow}{\def\janbert@arrow{#1}}
 \define@key{janbertdims}{color}{\def\janbert@color{#1}}
 \setkeys{janbertdims}{
   align=0cm,

   arrow=\bigg\downarrow,
   color=orange,
  }
  \newcommand\underbox[3][]{%
    \begingroup
      \setkeys{janbertdims}{#1}% the current values
      \underset{\mathclap{\substack{\janbert@arrow \[0.1cm] 
      \tikz\node[draw=\janbert@color, align=center, rounded corners]{#2}; \janbert@align }}}{#3} 
    \endgroup
  }
  \newcommand\overbox[3][]{%
   \begingroup
     \setkeys{janbertdims}{#1}% the current values
     \overset{\mathclap{\substack{\tikz\node[draw=\janbert@color, align=center, rounded corners]{#2}; 
     \janbert@align \[0.1cm]    \janbert@arrow 
    }}}{#3} 
   \endgroup
  }
 \makeatother
\begin{document}
 $$
   a_{n+2}=b_n a_n \underbox[align=2cm]{ $a_n=b_{n-2}a_{n-2}$}{=} 
   b_n b_{n-2} a_{n-2}\underbox[align=0.6cm,arrow={\Bigg\downarrow}]{$a_{n-2}=b_{n-4}a_{n-4}$}{=} 
   \dots 
   \overbox[align=4.25cm]{this process concludes when \[0.1cm] $n-2k$ is $0$ ou $1$}{=} 
   \begin{cases} 
     b_n b_{n-2}\dots b_{2} b_0  a_0 
     \overbox{$a_0=0$}{=}0 & \text{if $n=2k$} \
     b_n b_{n-2} \dots b_3b_1 a_1
     \underbox{$a_1=1$ }{=} b_n b_{n-2} \dots b_3b_1 & \text{if $n=2k+1$}
   \end{cases}
 $$
 \end{document}

Mico · Answer 1 · 2020-09-27T15:39:04.467

I suggest you encase the first part of the first row in the cases environment in a \smash[t]{...} wrapper and the first part of the second row in a \smash[b]{...} wrapper. This works because the bubbles to the left of the cases environment exceed the height and depth of the bubbles inside the cases environment.

I would also replace 3 of the 4 instances of \dots with \dotsb.

Oh, and don't use $$ to initiate and terminate displaymath mode in a LaTeX document. For more information on this subject, please see the posting Why is \[ ... \] preferable to $$ ... $$?

\documentclass[preview,,border=2pt]{standalone}
 \usepackage{mathtools}
 \usepackage{tikz}
 \setlength{\textwidth}{15cm}
\makeatletter
 \define@key{janbertdims}{align}{\def\janbert@align{\hspace{#1}}}
 \define@key{janbertdims}{arrow}{\def\janbert@arrow{#1}}
 \define@key{janbertdims}{color}{\def\janbert@color{#1}}
 \setkeys{janbertdims}{
   align=0cm,

   arrow=\bigg\downarrow,
   color=orange,
  }
  \newcommand\underbox[3][]{%
    \begingroup
      \setkeys{janbertdims}{#1}% the current values
      \underset{\mathclap{\substack{\janbert@arrow \[0.1cm] 
      \tikz\node[draw=\janbert@color, align=center, rounded corners]{#2}; \janbert@align }}}{#3} 
    \endgroup
  }
  \newcommand\overbox[3][]{%
   \begingroup
     \setkeys{janbertdims}{#1}% the current values
     \overset{\mathclap{\substack{\tikz\node[draw=\janbert@color, align=center, rounded corners]{#2}; 
     \janbert@align \[0.1cm]    \janbert@arrow 
    }}}{#3} 
   \endgroup
  }
 \makeatother
\begin{document}
 [
   a_{n+2}=b_n a_n \underbox[align=2cm]{ $a_n=b_{n-2}a_{n-2}$}{=} 
   b_n b_{n-2} a_{n-2}\underbox[align=0.6cm,arrow={\Bigg\downarrow}]{$a_{n-2}=b_{n-4}a_{n-4}$}{=} 
   \dots 
   \overbox[align=4.25cm]{this process concludes when \[0.1cm] 
                          $n-2k$ is $0$ ou $1$}{=} 
   \begin{cases} 
     \smash[t]{b_n b_{n-2}\dotsb b_{2} b_0  a_0 \overbox{$a_0=0$}{=}0} 
         & \text{if $n=2k$} \
     \smash[b]{b_n b_{n-2} \dotsb b_3b_1 a_1 \underbox{$a_1=1$ }{=} 
             b_n b_{n-2} \dotsb b_3b_1} 
         & \text{if $n=2k+1$}
   \end{cases}
 ]
 \end{document}

I wanted to use the answer of the problem to improve the command \overbox and \underbox that I implemented. I see that this is not possible as I did because affect to part that are not include in the argumentos of the command. Anyway — Barrientos, Sep 15 '20 at 00:54
@Barrientos - Glad you found my answer useful. :-) About modifying \overbox and \underbox: I'm not sure I understand what you're trying to achieve. Are you maybe trying to include \smash in one of the arguments of these commands? — Mico, Sep 15 '20 at 04:34
Yes it is. I wanted to include in the definition of \overbox some structure that allows me to control this type of problem. Another typical problem, that \overbox has, is the following: when it is located in a splitting equation the box appear in between of the lines. Sometimes this is not the effect that one want to get. I use your answer \smash[t]{...} to control this. But I do not know if one could implement some structure inside of the definition of \overbox that could provide a priori this control. — Barrientos, Sep 16 '20 at 23:48

Align a box in the formula

1 Answers1