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}invec{Z}_{xy},forall xinvec{x},forall yinvec{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?

Karl Ove Hufthammer · Accepted Answer

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

begin{equation}
  d(vec{x},vec{y}) =
  sum_{substack{Z_{xy}invec{Z}_{xy}
                  forall xinvec{x}
                  forall yinvec{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}invec{Z}_{xy}
                              forall xinvec{x}
                               forall yinvec{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}invec{Z}_{xy},
          forall xinvec{x},
          forall yinvec{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.

Ian Thompson · Answer

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

VIVID · Answer

I think it can be done without substack as
sum_{a=b  b=c  c=a}

wich results in

Separate long math text under sum symbol into different lines?

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