boldbracketsequations
I want to write an equation with bold extended brackets.
For instance in:
\left[ \frac{1}{2} \vec{\nabla} \times \vec{v} \right]
I want \left[ and \right] to be bold. I already tried https://tex.stackexchange.com/a/99286/15659 nothing is doing the trick.
I think that some information about the actual expressions might be useful to decide which structure(s) would be best to use; in its present form (using arrays for the two subgroups), one could use a cases environment for the outer brace:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{cases}
\left\{
\begin{array}{lcl}
1 \\
2
\end{array}
\right. \\
\left\{
\begin{array}{lcl}
3 \\
4
\end{array}
\right.
\end{cases}
\end{equation}
\end{document}
Here's another option, using a gather environment for the whole structure, and nested arrays:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{gather}
\left\{
\begin{array}{@{}l@{}}
\left\{
\begin{array}{lcl}
1 \\
2
\end{array}
\right. \\
\left\{
\begin{array}{lcl}
3 \\
4
\end{array}
\right.
\end{array}
\right.
\end{gather}
\end{document}
By the way, your code has some typos like \end{array{lcl}.
\documentclass{beamer}
\usepackage{amsmath}
\begin{document}
\begin{frame}{Title}
\begin{align*}
0 &\leq 2 \sigma - (k+2) \\
\boldsymbol{\frac{k}{2} + 1 }&\boldsymbol{\leq \sigma}
\end{align*}
\end{frame}
\end{document}
Not too elegant ... but works.
Best Answer
You can do
It does not work with
\left,\rightwith the default cm fonts as there is no bold extension font (for large brackets) in that font set. However extended brackets are not needed here.