I'm writing on overleaf.com and tried \tag#, where is the number of the equation. Here's exactly what I did: $$E=\vec p^2+m^2 \tag1$$. Any ideas on how to do it?
PS: I should say I'm not positive wether it really is math mode. I'm just repeating everything I see on physics and math stackexchange pages and this is the first time it doesn't work.
Physics.SE and Math.SE both use MathJax to render their math content. This requires the use of the following syntax
$$
<math>
$$
for display math content, and $<math>$ for inline math content. While this does work for MathJax, you need a little more in order to write an equation with tags using Overleaf:
\documentclass{article}
\usepackage{amsmath}
\setlength{\parindent}{0pt}% Just for this example
\begin{document}
Display equation without label:
\[
f(x) = ax^2 + bx + c
\]
The above is similar to
\begin{equation*}
f(x) = ax^2 + bx + c.
\end{equation*}
Display equation with label to \eqref{eqn:quadratic1}:
\begin{equation}
f(x) = ax^2 + bx + c \label{eqn:quadratic1}
\end{equation}
Display equation with custom label to \eqref{eqn:quadratic2}:
\begin{equation}
f(x) = ax^2 + bx + c \tag{quadratic} \label{eqn:quadratic2}
\end{equation}
\end{document}
