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how can I make a big summation sign ?

‎\begin{align}‎
‎\cos x‎ ‎=‎ ‎‎\sum\limits‎_‎{n=0}^{‎\‎‎infty‎} ‎\frac{(ix)^2n‎}{(2n)!}‎‎
\end{align}‎

this is the code which I'm using for summation

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3  
Isn't that big enough? By the way, you don't need \limits in an align; and you don't need it elsewhere, either. –  egreg Jul 11 '11 at 14:06
    
And you don't need \dfrac in an align, \frac is OK. –  Leo Liu Jul 11 '11 at 14:13
    
I agree with @egreg: using a consistent size of \sum could be better than varying the size manually depending on the following expression. Additional fine-tuning can be ok, however I would compare such equation design in good books before. –  Stefan Kottwitz Jul 11 '11 at 15:08
    
thank you very much , that's right but I'm typing my article just in the way my professor asked me ! –  Negin Jul 11 '11 at 15:16

4 Answers 4

up vote 24 down vote accepted

You can use the command \mathlarger of the relsize package. It increases the size and it can be nested. For example:

\documentclass{article}
\usepackage{amsmath}
\usepackage{relsize}
\begin{document}
\begin{equation}‎
‎‎\cos x‎ ‎=‎ \mathlarger{\mathlarger{‎‎\sum}}_{n=0}^{\infty‎}\frac{(ix)^2n}{(2n)!}‎‎
\end{equation}‎
\end{document}

equation with bigger sum symbol

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thank you Stefan that was very useful –  Negin Jul 11 '11 at 15:18

Well, there's always a graphicx solution.

\nsum[<optional>]

The command \nsum resizes the \sum symbol by the factor 1.4 (fits OP's fraction). It does take an optional argument to give another factor.

I admit that the \raisebox factor calculation is a bit sketchy as it uses the text style \sum symbol for the calculation but for cmr it's a good fit and I'm too lazy to take a closer look at the needed dimension calculation.

\resum{<arg>}

Another more dynamic approach would be to take the dimensions of the \sum's terms and resize the \sum sign so that it fits their vertical dimensions. To only specify the therms once the \resum also defines (overwrites) \s so that one can use

\resum{<what to sum>}_{foo}^{bar} \s

(I also tried to use \resizebox that takes the dimension instead of a factor, but I couldn't quite work out how to use \resizebox and \resizebox*.)

The second screenshot (produced by the lua-visual-debug package shows why \resum is in my eyes unstable and shouldn't be used. (But I don't think that a bigger \sum sign is even needed, anyway)

Code

\documentclass{article}
\usepackage{amsmath}
%\usepackage{lua-visual-debug} requires LuaTeX (used only for demonstration purposes)
\usepackage{graphicx}
\usepackage{calc}
\newlength{\depthofsumsign}
\setlength{\depthofsumsign}{\depthof{$\sum$}}
\newlength{\totalheightofsumsign}
\newlength{\heightanddepthofargument}

\newcommand{\nsum}[1][1.4]{% only for \displaystyle
    \mathop{%
        \raisebox
            {-#1\depthofsumsign+1\depthofsumsign}
            {\scalebox
                {#1}
                {$\displaystyle\sum$}%
            }
    }
}
\newcommand{\resum}[1]{%
    \def\s{#1}
    \mathop{
        \mathpalette\resumaux{#1}
    }
}

\newcommand{\resumaux}[2]{% internally
    \sbox0{$#1#2$}
    \sbox1{$#1\sum$}
    \setlength{\heightanddepthofargument}{\wd0+\dp0}
    \setlength{\totalheightofsumsign}{\wd1+\dp1}
    \def\quot{\DivideLengths{\heightanddepthofargument}{\totalheightofsumsign}}
    \nsum[\quot]%
}

% http://tex.stackexchange.com/a/6424/16595
\makeatletter
\newcommand*{\DivideLengths}[2]{%
  \strip@pt\dimexpr\number\numexpr\number\dimexpr#1\relax*65536/\number\dimexpr#2\relax\relax sp\relax
}
\makeatother

\begin{document}
\begin{equation}
  \cos x = \nsum_{n=0}^\infty \frac{(ix)^2n}{(2n)!}
\end{equation}
\begin{equation}
  \rlap{\rule[.57ex]{.55\linewidth}{.1pt}}\rlap{\rule{.55\linewidth}{.1pt}}
  \sum_{n=1}^\infty \nsum_{1.4} \nsum[2]_2 \nsum[2.5]_{2.5} \nsum[3]_{n=1}^\infty \nsum[3]_{\displaystyle n=1}^{\displaystyle \infty}
\end{equation}
\begin{equation}
     \cos x = \resum{\frac{(ix)^2n}{(2n)!}}_{n=0}^\infty \s \quad \resum{\frac{1}{2}}_{n=0}^\infty\s \quad \resum{\frac{\strut 1}{\strut 2}}_{n=0}^\infty\s 
\end{equation}‎
\end{document}

Output

Main output

A closer look at how \resum does (not) work

a closer look

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If you're willing to use the Mathtime Professional II font package, you could use its \xl\sum, \XL\sum, \XXL\sum etc commands to get larger summation symbols.

enter image description here

\documentclass{article}
\usepackage{times,mtpro2}
\begin{document}
The summation symbols below are generated by 
\texttt{\textbackslash sum} and by
\texttt{\textbackslash xl\textbackslash sum}, 
\texttt{\textbackslash XL\textbackslash sum}, and 
\texttt{\textbackslash XXL\textbackslash sum} of the
\textbackslash{mtpro2} package. There is also
\texttt{\textbackslash XXXL\textbackslash sum}, 
but it's not shown here.
\[
\sum_{i=1}^T \frac{1}{2^{-i}}  \qquad 
\xl\sum_{i=1}^T \frac{1}{2^{-i}} \qquad 
\XL\sum_{i=1}^T \frac{1}{2^{-i}} \qquad 
\XXL\sum_{i=1}^T \frac{1}{2^{-i}}
\]
\end{document}

Do note, though, that (i) these fonts aren't free (though not exactly expensive either) and (ii) they are in the "Times Roman" font style, which may or may not be to your liking.

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Sadly, the answer given above by Stefan Kottwitz doesn't work will all fonts. I use the lmodern fonts so that underscores are actual underscores in PDF files (necessary, for example, for cut & paste of included URL's):

\usepackage{lmodern}

As soon as I include this \usepackage line the output given above turns into

âĂŐâĂŐâĂŐ cos xâĂŐâĂŐ = âĂŐâĂŐâĂŐâĂŐ
1 (ix)2 n âĂŐâĂŐ (2n)! n=0 (1)

(This isn't exactly what it looks like -- this is a text cut & paste from the PDF output; but you get the idea.)

When I read the documentation for the relsize package the authors state that \mathlarger{} and \mathsmaller{} are hacks that won't work in many cases.

In all, this appears not to be a good solution. I found this discussion because I also think the default summation sign is too small; e.g. in this example:

\begin{equation*}
P(X \ge 8) = \mathlarger{\sum}_{k=8}^{10} \binom{10}{k} \left(\frac{1}{2}\right)^{10} =  0.0546875
\end{equation*}

However, I was surprised to find that there doesn't seem to be any other way to make the summation sign larger. It would be nice to have something like \dsum (similar to \dfrac{}{} and \dbinom{}{})

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2  
Sadly, this does not answer the question... –  Werner Nov 6 '12 at 20:24
    
Which input encoding system do you use? –  Mico Nov 6 '12 at 20:49
4  
I understand that this doesn't answer the question, but wanted to provide a correction to the accepted answer. I would have preferred to post my response as a comment to that answer, but didn't have the option because of lack of reputation. I tried this solution previously, found it didn't work and only just now had time to track down why; something likely to be useful to other users (particularly since this article comes up as the first hit on google when searching for "latex make summation symbol larger". The moderator can recast my non-answer as a commment to Stefan Kottwitz' answer. –  pgoetz Nov 6 '12 at 22:41

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