5

have often seen in texts on Lie groups a capital A but without the middle bar but how to make such in Latex ? enter image description here

2
17

That's an upper-case lambda, which can be obtained in math mode as \Lambda:

\documentclass{article}
\usepackage{amsfonts}
\begin{document}
$\gamma \colon \mathfrak{g} \to \Lambda^2 \mathfrak{g}$
\end{document}

enter image description here

4

In the case of Lie algebras you may actually be looking at \wedge or \bigwedge for the alternating algebra.

\documentclass{article}
\usepackage{amsfonts}
\begin{document}
$\gamma \colon \mathfrak{g} \to \bigwedge^2 \mathfrak{g}$
\end{document}

I would submit that $$\gamma \colon \mathfrak{g} \to \bigwedge^2 \mathfrak{g}$$ is the "right" way in this case, but there are people out there who use the $\Lambda$.

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