Equation error in Latex table - latex

Question

I'd like to create a table with two parameters and the corresponding equation.
Somehow Latex gives me the error:
'! Missing $ inserted.'
and I don't know what that means...
\begin{table}[H]
\centering
\caption{XXX}
\label{tab:XXX}
\begin{adjustbox}{max width=\textwidth}
\begin{tabular}{c c}
\hline\hline
\vspace{0.25cm}
Parameter & Mathematical Equation \\ \hline
\vspace{0.25cm}
Standard Deviation & \sigma=\sqrt{\frac{\sum(X-\mu)$^2$}{N}} \\
\vspace{0.25cm}
Variance & \sigma^2=\frac{\sum(X-\mu)$^2$}}{N} \\
\\ \hline
\end{tabular}
\end{adjustbox}
\end{table}

Answer 1

In TeX, $ signs enclose inline math code (see here). Your example already uses it around ^2. However, you also use other macros that are only valid inside a math environment, such as e.g. \sigma. Hence, you should surround all macros that require math mode with $ signs:
\begin{table}[H]
\centering
\caption{XXX}
\label{tab:XXX}
\begin{adjustbox}{max width=\textwidth}
\begin{tabular}{c c}
\hline\hline
\vspace{0.25cm}
Parameter & Mathematical Equation \\ \hline
\vspace{0.25cm}
Standard Deviation & $\sigma=\sqrt{\frac{\sum(X-\mu)^2}{N}}$ \\
\vspace{0.25cm}
Variance & $\sigma^2=\frac{\sum(X-\mu)^2}{N}$ \\
\\ \hline
\end{tabular}
\end{adjustbox}
\end{table}
If you prefer not to mix TeX and LaTeX commands, consider using \(...\) instead of $...$.