\documentclass{article}
\usepackage{amsmath}
\begin{document}
\section{Named dots}
$A,B,\dotsc,Z$

$A + B +\dotsb +Z$

$A B +\dotsm Z$

$\int \dotsi dx$

$A B \dotso Z$


\section{Automatic dots}
$A,B,\dots,Z$

$A,B,\dots Z$

$A + B +\dots +Z$

$A + B +\dots \boldsymbol{+}Z$

$A + B +\dots -Z$

$A = B =\dots =Z$

$\int \dots dx$

$A B \dots Z$

$A B \dots$

$\frac{A \dots}{B\dots}$

\end{document}