Know you want to $\LaTeX$ up a summation symbol, but don’t know the command to use? Don’t stress! Head to DeTexify and draw what you’re looking for:
Notice that there are a few results. The one we want is the highlighted one, which tells us to use \sum within math mode. For example, to produce
$$\sum_{j = 1}^{n} \mathrm{Col}_j(A)$$ we use the code
$\sum_{j = 1}^{n} \mathrm{Col}_j(A)$ %inline version
\[
\sum_{j = 1}^{n} \mathrm{Col}_j(A) %display version
\]
(we need enclosing $ ... $ or \[ ... \] because \sum only works in math mode).
Now let’s say you actually weren’t interested in the summation symbol but really wanted to typeset a fancy greek Sigma like in the third result. You’ll notice an extra \usepackage parameter, meaning that this symbol isn’t included in $\LaTeX$ by default. That’s okay. Just add the code \usepackage{upgreek}
to your preamble so that you can use $\Upsigma$ in the body of your $\LaTeX$ document:
\documentclass{article}
\usepackage{amsmath,amssymb,amsthm} % if you're using one of my templates, you have these
\usepackage{upgreek} % you'll have to add this yourself
...
\begin{document}
Here is my fancy $\Upsigma$!
\end{document}
Here’s a little homework: use DeTexify to figure out what package you need to typeset $\eth$.