You can see there is an unwanted white space at the last term below.
\documentclass[border=12pt]{standalone}
\usepackage{amsmath}
\begin{document}
$
\lefteqn{\textstyle\overbrace{\phantom{1+2+3+4}}^{\alpha}}1+2+
\underbrace{3+4+
\lefteqn{\textstyle\overbrace{\phantom{5+6+7}}^{\beta}}5+6}_{\gamma}+7
$
\end{document}
What causes the white space? And how to remove it?
The boxing interferes with the math spacing, if you make the phantoms visible you see what is being braced:
If you correct for the mathop spacing you can make them align exactly:
\documentclass[border=12pt]{article}
\usepackage{amsmath,color}
\begin{document}
$
\lefteqn{\textstyle\overbrace{\phantom{1+2+3+4}}^{\alpha}}1+2+
\underbrace{3+4+
\lefteqn{\textstyle\overbrace{\phantom{5+6+7}}^{\beta}}5+6}_{\gamma}+7
$
\def\xphantom#1{\hbox{{\color{red}$#1$}}}
$
\lefteqn{\textstyle\overbrace{\xphantom{1+2+3+4}}^{\alpha}}1+2+
\underbrace{3+4+
\lefteqn{\textstyle\overbrace{\xphantom{5+6+7}}^{\beta}}5+6}_{\gamma}+7
$
$\setbox0\hbox{$3+4$}
\overbrace{1+2+3+4}^{\alpha}
\kern-\wd0\!
\underbrace{3+4+5+6}_{\gamma}
\kern-\wd0\!
\overbrace{5+6+7}^{\beta}
$
\end{document}
See the discussion in
How to fix the math-spacing when a lot of \phantom, \mathrlap, etc. are used?
I've just released a new package designed to solve just this problem! It is called underoverlap.
Here's how I would then set your example:
$
\UOLoverbrace{1+2+}[3+4]^\alpha \UOLunderbrace{+}[5+6]_\gamma \UOLoverbrace{+7}^\beta
$
It needs no duplication in your source code and spacing is automatically correct in most cases (mostly thanks to David's help in the question he links to in his answer.)
