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How can i write my own math operator with limits? I want it to look like: \sum\limits_{e=1}^{m} but with a capital A (if possible bigger than the normal text) instead of the sum. Thanks for the help!

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Thanks for all the answers! really quick, really precise!! –  user6776 Jul 19 '11 at 8:35
    
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5 Answers 5

\newcommand{\opA}{\mathop{\vphantom{\sum}\mathchoice
  {\vcenter{\hbox{\huge A}}}
  {\vcenter{\hbox{\Large A}}}{\mathrm{A}}{\mathrm{A}}}\displaylimits}

In this way the "A" will be as large as the \sum symbol.

enter image description here

For a geometric symbol, it's perhaps better just to scale it, instead of guessing the size.

\documentclass{article}
\usepackage{amsmath}
\usepackage{graphicx}

\makeatletter
\DeclareRobustCommand{\bigstar}{%
  \mathop{\vphantom{\sum}\mathpalette\bigstar@\relax}\slimits@
}

\newcommand{\bigstar@}[2]{%
  \vcenter{%
    \sbox\z@{$#1\sum$}%
    \hbox{\resizebox{.9\dimexpr\ht\z@+\dp\z@}{!}{$\m@th\star$}}%
  }%
}
\makeatother

\begin{document}
\[
\sum_{i=1}^n\bigstar_{i=1}^n x_i
\qquad
\textstyle
\sum\bigstar_{i=1}^n x_i
\qquad
\scriptstyle
\sum\bigstar_{i=1}^n x_i
\qquad
\scriptscriptstyle
\sum\bigstar_{i=1}^n x_i
\]
\end{document}

enter image description here

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I'd like to do the same but using \star. What are the 4 sizes used in the choice? I'd like to use \LARGE\star. Is there different sizes for display or inline math? –  Sigur May 11 at 13:48
    
@Sigur I added the code for the big star; I used just 90 percent of the size of a summation symbol, because the star has large overshoot. –  egreg May 11 at 15:21
    
thanks so much. Amazing and beautiful. Since I like to use \textstyle size in displayed mode (the \sum in displaystyle is too big in my opinion) should I use \vphantom{\textstyle\sum}? –  Sigur May 11 at 15:32
    
@Sigur Just use \mathop{\textstyle\bigstar} –  egreg May 11 at 15:37
    
sorry. Use it where? In the text body? If I well understood, you made a box with the same height as the one produced by \sum. But every time I use \sum in display mode I use \textstyle and \limits o make the summation smaller. I'd like to define the star symbol in such a way that I don't need to use \textstyle inside display mode. Simply \[ \bigstar_{i=1}^n\] with the same size as \[ \textstyle \bigstar\limits_{i=1}^n\]. –  Sigur May 11 at 15:43

Use the \DeclareMathOperator* command provided by the amsmath package. Example:

\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator*{\Aop}{A}
\begin{document}
\[ \Aop^a_b \]
\end{document}
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Use \mathop to turn A into a "math operator", and then use \limits to do sum-style limits:

 $$\mathop{A}\limits_{e=1}^{m}$$

Do you really want A, or do you want \bigwedge?

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\documentclass{article}
\usepackage{amsmath}
\def\Aop{\operatornamewithlimits{%
  \mathchoice{\vcenter{\hbox{\huge A}}}
             {\vcenter{\hbox{\Large A}}}
             {\mathrm{A}}
             {\mathrm{A}}}}
\begin{document}

\[ \Aop^a_b \sum_a^b \]

$ \Aop^a_b \sum_a^b $

\end{document}

enter image description here

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egreg's answer is fine but works poorly for existing mathematical symbols. I recommend the following:

\newcommand{\operator}[1]{\mathop{\vphantom{\sum}\mathchoice
{\vcenter{\hbox{\huge $#1$}}}
{\vcenter{\hbox{\Large $#1$}}}{#1}{#1}}\displaylimits}

To define an operator 'A', you can now type

\newcommand{\opA}{\operator{\mathrm{A}}}

But you can also define an operator Theta:

\newcommand{\opTheta}{\operator{\Theta}}

Just remember that in the argument you pass to \operator, you are in math mode.

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