Bracing fraction numerator or denominator to the left or right

Question

I have a fraction with further fractions in both the numerator and denominator. I wish to label the fractions in the numerator and denominator with braces to the right. I would like these braces to span the full height of the fractions but I would not like them to extend the fraction line any further.

I have tried \frac{a}{\left.\frac{b}{c}\right}} which achieves the former but not the latter and I have tried \frac{a}{\frac{b}{c}\mathrlap{\}}} which achieves the latter but not the former, since I can't put one half of the \left \right pair inside \mathrlap and the other half outside.

Answer 1

Perhaps what you are looking for is a combination of \mathrlap and \smash which is what is incorporated in the \BraceRight and \BraceLeft macros.

However, as HendrikVogt pointed out, this requires that there be manual spacing added afterwards if there is text that follows it. To alleviate this issue I have defined two more macros \FracBraceBottomRight and \FracBraceBottomLeft which also add the approriate \hptantom after or before the fraction.

One downside of this this is that you need a different macro in case you desire further options (such as optional braces around either side of the numerator. This can be fixed with a better interface to enable/disable the appropriate brace, but I think that that is more of a question of coming up with the better UI.

References:

• The \BracKern is to adjust for the horizontal space inserted by the \left., \right. as per How to compute exact width added by \left. \right

Code:

\documentclass{article}
\usepackage{mathtools}

\newcommand*{\BraceRight}[1]{#1\mathrlap{\smash{\left.\vphantom{#1}\right\}}}}%
\newcommand*{\BraceLeft}[1]{\mathllap{\smash{\left\{\vphantom{#1}\right.}}#1}%

% https://tex.stackexchange.com/questions/54023/how-to-compute-exact-width-added-by-left-right
\newcommand{\BracKern}{\kern-\nulldelimiterspace}
\newcommand*{\FracBraceBottomRight}[2]{%
    \frac{#1}{#2\mathrlap{\smash{\left.\BracKern\vphantom{#2}\right\}}}}%
    \hphantom{\left.\vphantom{#2}\right\}}%
}%
\newcommand*{\FracBraceBottomLeft}[2]{%
    \hphantom{\left\{\vphantom{#2}\right.\BracKern}%
    \frac{#1}{\mathllap{\smash{\left\{\vphantom{#2}\right.}}#2}%
}%

\begin{document}
Requires manual spacing:

$\frac{a}{\frac{b}{c}}$\quad%
$\frac{a}{\BraceLeft{\frac{b}{c}}}$\quad%
$\frac{a}{\BraceRight{\frac{b}{c}}}$

\bigskip
Auto spacing with \verb|\hphantom|:

$\FracBraceBottomRight{a}{\frac{b}{c}}$%
$\FracBraceBottomRight{a}{\frac{b}{c}}$%
$\FracBraceBottomLeft{a}{\frac{b}{c}}$%
$\FracBraceBottomLeft{a}{\frac{b}{c}}$
\end{document}