4

I'm trying to reproduce the following:

enter image description here

I tried to use tikzpicture and made something "by hand" (even knowing that that wasn't the better way), but I was not able to find a way to have the exactly result, especially in the middle of the text as the image above. My code:

\documentclass{article}
\usepackage{tikz-qtree}

\begin{document}
    \begin{tikzpicture}
    \Tree [.A \edge[roof]; {$\alpha_{1} \ldots \alpha_{n}$} ]
    \end{tikzpicture}
\end{enumerate}
\end{document}
5

With pure TikZ.

\documentclass{article}
\usepackage{tikz}

\begin{document}
\begin{enumerate}
 \item \dots
 \item For each subtree of the form 
   \begin{tikzpicture}[baseline=0.5em]
    \node (a){$\alpha_{1} \ldots \alpha_{n}$};
    \draw (a.90) + (0,0.3) node[above]{$A$} -- (a.30) -- (a.150) -- cycle;
    \end{tikzpicture}
   do \dots 
\end{enumerate}
\end{document}

enter image description here

5

Or, if you want an equilateral triangle:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric, positioning}
\newcommand{\mytri}[2]{%}
   \begin{tikzpicture}[baseline=(a.south)]
      \node[
        draw,
        regular polygon,
        regular polygon sides=3,
        text width=.2em
        ] (a) {};
         \node[above=0pt of a] {$#1$};
         \node[below=0pt of a] {$#2$};
    \end{tikzpicture}}

\begin{document}
\begin{enumerate}
\item \dots
 \item For each subtree of the form 
\mytri{A}{\alpha_{1} \ldots \alpha_{n}}
   do \dots 
\end{enumerate}
\end{document}

enter image description here

3

Without Tikz

\documentclass{article}
\usepackage{lmodern}

\begin{document}
\begin{enumerate}
 \item \dots
 \item For each subtree of the form 
   $\displaystyle\mathop{\triangle}_{\alpha_1\dots\alpha_n}^{A}$
   do \dots 
\end{enumerate}
\end{document}

enter image description here

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