why do I have a Missing $ inserted error in this document

Question

Can someone help me understand this error message and how to fix it? I don't understand why it thinks it needs to insert a $. The error message is given below.

\documentclass{article}
\usepackage{qtree}
\begin{document}

\[ \neg x_1 \wedge \neg x_2 \wedge x_3 \wedge \neg x_4 = 
 \Tree[.\wedge [.\wedge [.\wedge [.\wedge \top [.\neg x_1 ] ] [.\neg x_2 ] ] x_3 ] [.\neg x_4 ] ] \]

\end{document}

Here is the error message.

! Missing $ inserted.
<inserted text> 
                $
l.8 ...wedge [.\wedge [.\wedge \top [.\neg x_1 ] ]
                                                   [.\neg x_2 ] ] x_3 ] [.\n...

Answer 1

Probably the nodes of the tree is set in text mode (even if the tree is inside a math environment), so you need $ .. $ throughout.

\documentclass{article}
\usepackage{qtree}
\begin{document}

\[ 
\neg x_1 \wedge \neg x_2 \wedge x_3 \wedge \neg x_4 = 
 \Tree[.$\wedge$ 
        [.$\wedge$ 
          [.$\wedge$
            [.$\wedge \top$ [.$\neg x_1$ ] ]
            [.$\neg x_2$ ]
          ] $x_3$
        ] [.$\neg x_4$ ]
       ]
\]

\end{document}