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How can I make the following part of code:

\sum_{i=1}^n \sum_{j=1}^n w_{ij}

in normal size and not in subscript size?

\documentclass[a4paper]{article}
\usepackage[english]{babel}

\begin{document}

\begin{equation}
I = \frac{n}{\sum_{i=1}^n \sum_{j=1}^n w_{ij}}
\frac{\displaystyle\sum_{i=1}^n \sum_{j=1}^n w_{ij}(x_i - \bar{x})(x_j -
  \bar{x})}{\displaystyle\sum_{i=1}^n (x_i - \bar{x})^2}
\end{equation}

\end{document}
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2 Answers 2

up vote 3 down vote accepted

You should add a \displaystyle directive in the denominator of the first \frac expression:

\documentclass[a4paper]{article}
\usepackage[english]{babel}
\begin{document}

\begin{equation}
I = \frac{n}{\displaystyle\sum_{i=1}^n \sum_{j=1}^n w_{ij}}
    \frac{\displaystyle\sum_{i=1}^n \sum_{j=1}^n w_{ij}(x_i 
      - \bar{x})(x_j - \bar{x})}{\displaystyle\sum_{i=1}^n (x_i - \bar{x})^2}
\end{equation}
\end{document}

Addendum: Technically, without the displaystyle directive, the main elements of expressions in the numerator and denominator of a \frac command that's part of a displayed equation are not in "(sub)script" style but in "text style".

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as i interpret this question, you want the sums in the denominator of the first fraction to be the same size as those in the second fraction, but you want them to be shown with sub/superscripts, not with limits. add \displaystyle to that expression, and insert \nolimits after each \sum, as

{\displaystyle\sum\nolimits_{i=1}^n \sum\nolimits_{j=1}^n w_{ij}}

giving this result:

display with adjusted fraction

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Thank you. Now my formula looks nice. Now I'd like to add an explanation to it, and when I type $\bar{x}$ out of the equation environment, the symbol is not displayed. How can I do that? –  Costanza Sep 5 '12 at 13:22
    
@Costi -- i can't think of any reason that $\bar{x}$ wouldn't be shown in accompanying ordinary text, like a following paragraph. please be more specific about the form you want your explanation to take. –  barbara beeton Sep 5 '12 at 13:43
    
This is what I get when trying to run \bar{x} in normal text: ! Package amsmath Error: \bar allowed only in math mode. –  Costanza Sep 5 '12 at 15:32
    
@Costi -- yes, you need to enclose it in $...$. you said in your first comment that dollar signs were used. if it is in dollar signs and still gets that error message, then it will be necessary to see a working example, complete with document class and relevant packages. put this in a new question, as extended code is nearly indecipherable in a comment. –  barbara beeton Sep 5 '12 at 16:42
    
\documentclass{book} \usepackage{amsmath} \usepackage{fixltx2e} \usepackage[english]{babel} \usepackage[bitstream-charter]{mathdesign} \usepackage[T1]{fontenc} \begin{document} $\bar{x}$ \end{document} –  Costanza Sep 5 '12 at 16:59

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