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I have an equation in eqnarray environment. The equation has a sum notation with subscript and superscript. The problem is that the subscript and superscript appear on sideways of the sum notation which looks pretty ugly. I came to know about \displaystyle but it can be used only with inline equation.

How can I use such a command with the equation or eqnarray environment.

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\align doesn't work either. Just consider it an equation having only a sum notation with subscript and superscript. Both the scripts are appearing on the lower and upper right sides of the sum notation. –  user846400 Jul 17 '12 at 19:57
    
Actually, I want to place the \sum within a fraction but I don't like the way that it appears when compiled i.e. the N above it and the i = 1 below it appear as plain superscripts and subscripts respectively but I want them to be directly above and below the "sum of" sign on the top and bottom of the fraction. Normally, if you use \displaymath, then this works fine but I need the equation to be correctly numbered so I need to use it within the \equation environment. –  user846400 Jul 17 '12 at 20:04
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migrated from stackoverflow.com Jul 18 '12 at 11:16

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1 Answer

It may just be that you're using the environment incorrectly. This might be what you're after:

enter image description here

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{align}
  r    &= \sum_{i=k}^n\binom{n}{i}\binom{m-n}{n-i} \\
  f(x) &= \frac{\sum_{i=k}^n\binom{n}{i}\binom{m-n}{n-i}}{\binom{m}{n}} \\
  g(x) &= \frac{\displaystyle\sum_{i=k}^n\binom{n}{i}\binom{m-n}{n-i}}{\binom{m}{n}}
\end{align}
\end{document}

amsmath provides the align (numbered) environment, as well as \binom. Numerator and denominator of fractions in display math mode are normally converted to \textstyle. Therefore, it requires a verbatim switch to \displaystyle if needed.

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Yes. This is what I want. I have sum in the denominator. However, if I use \displaystyle as above, I do get the sum in right form, but I get some extra spacing before and after the equation. –  user846400 Jul 18 '12 at 7:25
2  
@user846400 eqnarray requires two & symbols: left hand side & = & right hand side. But it's better not to use eqnarray at all, favoring amsmath environments. –  egreg Jul 18 '12 at 11:21
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