The relevant code is
Code: Select all
\usepackage{amsmath}
\left[\frac{1}{\dfrac{1}{\sqrt{13}-3}-a_1}\right]
Code: Select all
\usepackage{amsmath}
\left[\frac{1}{\dfrac{1}{\sqrt{13}-3}-a_1}\right]
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Code: Select all
\documentclass{minimal}
\usepackage{amsmath}
\makeatletter
\newcommand*{\vertcent}{\mathpalette\vertc@nt}
\newcommand*{\vertc@nt}[2]{\vcenter{\hbox{$\m@th#1#2$}}}
\makeatother
\begin{document}
\[ a + \left[ \vertcent{\frac{1}{\dfrac{1}{\sqrt{13} - 3} - a_1}} \right] + b \]
\end{document}
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