Separate long math text under sum symbol into different lines?

Question

I have the following equation, where there is a lot of stuff under the sum symbol:

\begin{equation}
d(\vec{x},\vec{y})=
  \sum_{Z_{xy}\in\vec{Z}_{xy},\forall x\in\vec{x},\forall y\in\vec{y}} 
    f(Z_{xy})
\end{equation}

In the resulting document, I find it kind of hard to read. Is there a way to write the equation to make the result more readable, e.g. putting the stuff under the sum symbol on different lines?

Answer 1

You can use the \substack command from the amsmath package, like this:

\begin{equation}
  d(\vec{x},\vec{y}) =
  \sum_{\substack{Z_{xy}\in\vec{Z}_{xy}\\
                  \forall x\in\vec{x}\\
                  \forall y\in\vec{y}}}
        f(Z_{xy}) 
\end{equation}

However, the result still doesn’t look good, because of the extra spacing around the sum symbol:

To fix this, you can use the \mathclap command from the mathtools package, like this:

\begin{equation}
 d(\vec{x},\vec{y}) =
    \sum_{\mathclap{\substack{Z_{xy}\in\vec{Z}_{xy}\\
                              \forall x\in\vec{x}\\
                               \forall y\in\vec{y}}}}
          f(Z_{xy}) 
\end{equation}

But perhaps you might be happy using only \mathclap, and not \substack. The result looks good as long as the subscript is not too wide.

\begin{equation}
  d(\vec{x},\vec{y}) =
    \sum_{\mathclap{{Z_{xy}\in\vec{Z}_{xy},
          \forall x\in\vec{x},
          \forall y\in\vec{y}}}} f(Z_{xy})
\end{equation}

The mathtools package also have several other useful commands for typesetting mathematics, including more commands for improving the display of subscripts and superscripts. I very much recommend taking a look at its documentation.

Answer 2

Try the \substack command from the amsmath package, details of which are found here

Answer 3

[For those who are looking for MathJax way] it can be done without \substack as

\sum_{a=b \\ b=c \\ c=a}

wich results in

Answer 4

Using A \atop B. E.g:

\sum_{{n\in \mathbb{N}}\atop{n= even}}^{100} a_n