Text across several summation symbols

Question

I have a question on how to have the following in an equation.

Here is some working code to start with:

\documentclass[12pt]{article}

\begin{document}

\begin{equation}
P(X)=\sum_{X\in A}\cdots \sum
\end{equation}

\end{document}

Answer 1

Here is one way via declaring an operator

\documentclass[12pt]{article}
\usepackage{mathtools} %<-- loads, enhances amsmath
\DeclareMathOperator*{\manysum}{\sum\cdots\sum}

\begin{document}

\begin{equation}
P(X)=\manysum_{X\in A}(x_{ijklm})
\end{equation}
Also inline \(P(X)=\manysum_{X\in A}(x_{ijklm})\).
\end{document}

Answer 2

Use \underset from amsmath.

\documentclass[12pt]{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}
P(X)=\underset{X\in A}{\sum \cdots \sum}
\end{equation}

\end{document}

If this is a common occurrence in your document, you might consider creating it as a new operator with limits, which is discussed in How to create my own math operator with limits? and in percusse's answer to your question here.

Answer 3

A more general answer:

\documentclass[12pt]{article}
\usepackage{amsmath}

\makeatletter
\newcommand{\Multi@Sum}[1]{%
  \ifcase#1\relax\or
    \or
    \sum\sum\or
    \sum\sum\sum\else
    \sum\dots\sum
  \fi
}
\newcommand\ssum{\DOTSB\mathop{\Multi@Sum{2}}\slimits@}
\newcommand\sssum{\DOTSB\mathop{\Multi@Sum{3}}\slimits@}
\newcommand\sdotssum{\DOTSB\mathop{\Multi@Sum{-1}}\slimits@}
\makeatother

\begin{document}

\begin{equation}
\sum_{x\in A}\quad
\ssum_{x\in A}\quad
\sssum_{x\in A}\quad
\sdotssum_{x\in A}
\end{equation}

\end{document}

This will respect the option nosumlimit or sumlimit (default) given to amsmath.

You may want to change the \sum\dots\sum line to

\sum\!\cdots\!\sum

getting, for \sdotssum_{x\in A} the output