Math & Science ⇒ \underbrace at a strange place

[bbasic]

Take a look at the following code:

Code: Select all

$$\begin{array}{rr|r|c|l}
vd_2:&\raisebox{0pt}[0pt]{\makebox[0pt][l]{$\overbrace{\phantom{b_1\dots b_{n-l_{k-1}-2k}\hspace{2\arraycolsep}b_{n-l_{k-1}-2k+1}\dots b_{n-k-1}b_{n-k}\hspace{2\arraycolsep}c^k}}^v$}}b_1\dots b_{n-l_{k-1}-2k}&b_{n-l_{k-1}-2k+1}\dots b_{n-k-1}b_{n-k}&c^k&d_2\\
\mathrm{PW}(w):&\dots w_r c^{r+1-k}&c^k\raisebox{0pt}[0pt][0pt]{\makebox[0pt][l]{$\underbrace{\phantom{w_{k-1}\hspace{4.17em}\hspace{2\arraycolsep}c^k\hspace{2\arraycolsep}\widetilde{w_{k-1}}\dots}}_{w_r}$}}w_{k-1}\hspace{2em}\hspace{25.5pt}&c^k&\widetilde{w_{k-1}}\dots\dots
\end{array}$$

I would like to draw the \underbrace in a more elegant way. The main problem is how to make sure that $c^kw_{k-1}$ is in the dead center of the column. In the proposed code, it has been done artifically: the empty space of length 4.17em is added to the right of $c^kw_{k-1}$, where the value 4.17em is obtained by trial-and-error. This works nicely in 12pt, but if the font size is changed, it slides off the center. Even if there were not a problem with changing the font size, it would seem logical that there is a nicer way to accomplish this effect, instead of guessing the value 4.17em (or whichever works for the particular font size) until a good enough pixel-precision is reached.

Thank you all for the suggestions.

[Stefan Kottwitz]

This code is very hard to read. That's why I can understand that nobody answered. It's recommendable to use more spaces on math mode code, so the code can be read by humans.

For braces in very complex expressions, such as this array, we could use TikZ for marking places and for drawing braces from one mark to another. Here's an example: Highlighting elements in matrices with TikZ.

Your example with such a \tikzmark, rewritten to be nicer to read and without all those \raisebox, \makebox and \hspace commands:

Code: Select all

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.pathreplacing}
\newcommand{\tikzmark}[1]{\tikz[overlay,remember picture,baseline=(#1.base)] \node (#1) {\strut};}
\begin{document}
\[
  \begin{array}{rr|c|c|l}
    vd_2:
      &\tikzmark{topleft}b_1\dots b_{n-l_{k-1}-2k}
      &  b_{n-l_{k-1}-2k+1}\dots b_{n-k-1}b_{n-k}
      & c^k \tikzmark{topright}
      & d_2 \\
    \mathrm{PW}(w):
      & \dots w_r c^{r+1-k}
      & \tikzmark{bottomleft} c^k w_{k-1}
      & c^k
      &\widetilde{w_{k-1}}\dots\dots\tikzmark{bottomright}
  \end{array}
\]
\tikz[overlay, remember picture, decoration={brace, amplitude=10pt}] {
  \draw[decorate,thick] (topleft.north) -- (topright.north)
      node [midway,above=10pt] {$v$};
  \draw[decorate,thick] (bottomright.south) -- (bottomleft.south)
      node [midway,below=10pt] {$w_r$};
}
\end{document}

Output:

braces.png (7.52 KiB) Viewed 4671 times

TikZ is a graphics package for LaTeX with excellent capabilities, even if we use it here just for a small purpose. It builds on pgf. Have a look at the TikZ manual if you are interested.

Stefan