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I' looking for a symbol rendering the connected sum of $n$ topological spaces (or other quite strange topological operations) in a "product-like" manner... What For example in writing

$$
{\Large{*}}_{i=1}^n G_i
$$

for the free product of n groups. How can I render this with the # operation in topological spaces? (Yes, I'm studying Hatcher's AT...)

Edit: (Some mind-reading clarification by Willie)

The OP is not asking about how to use the * or \ast symbols, if I read him correctly. He is asking the following question:

How to define a (possibly variable size) math operator like \sum and \prod where the operator symbol is the octothorpe #, such that for in-line maths the upper and lower limits follow next to the symbol, and in \displaystyle with the upper and lower limits above and below the symbol?

This seems to be the referred part of Hatcher’s AG:

definition from Hatcher

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It would be helpful if you supplied some information on what the result should look like. Should we assume you mean a star as used on the third math line under "Presentation" here? en.wikipedia.org/wiki/Free_product –  Tomas Lycken Nov 7 '10 at 13:13
    
Possible duplicate: tex.stackexchange.com/questions/23432/… –  Mechanical snail Dec 2 '12 at 1:34
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5 Answers 5

up vote 11 down vote accepted

The \mathop{..} command from amsmath makes its argument behave like a "large operator".

 \documentclass{article}
 \usepackage{amsmath}
 \usepackage{relsize}

 \newcommand{\bigstar}{\mathop{\Huge \mathlarger{\mathlarger{*}}}}

 \begin{document}

 Display:

 \[ \bigstar_{x}^{y} \]

 Inline: \( \bigstar_{x}^{y}\)


 \end{document}
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2  
\mathop is a TeX primitive, not a amsmath command. –  morbusg Dec 16 '10 at 10:14
    
Thanks for the correction. I'm not sure why I thought that... perhaps I learned about it from the AMS math manual or something like that. –  frabjous Dec 16 '10 at 17:06
1  
@frabjous -- two points: \mathop is indeed a primitive; it's used by amsmath (actually, amsopn) in the definition of \DeclareMathOperator. and the symbol shown in the edit to the question is \sharp, not a star. –  barbara beeton Dec 30 '11 at 14:00
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I'm not sure if the below code does what you want. At first I had the relsize package in mind, but it scaled the asterisk not big enough. So I come up with a possible solution that uses the graphicx package to do the job. The new operator needs at least amsmath (in this case loaded by mathtools) for its declaration.

\documentclass[11pt,a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}   % loads »amsmath«
\usepackage{graphicx}

\DeclareMathOperator*{\name}{\raisebox{-0.6ex}{\scalebox{2.5}{$\ast$}}}

\begin{document}
  \[
    \name_{i=1}^n G_i
  \]
\end{document}

Since I didn't know how you want to call the new operator, I simply chose a trivial name. Rename as you like.

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I am pretty sure the OP wants the # symbol instead of the \ast. Otherwise your answer is probably what he is looking for. –  Willie Wong Nov 7 '10 at 14:46
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You have some good answers for the procedure. The connected-sum symbol can be found in the mathabx package as \hash or \varhash. See “How to look up a math symbol?”.

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Detexifiy doesn't give me neither \hash nor \varhash, but I am very grateful to everybody –  tetrapharmakon Nov 7 '10 at 17:31
    
@tetrapharmakon: I didn't use detexify either. I just opened the symbols-letter.pdf and searched for "hash". At times I've just paged through it, too. –  Matthew Leingang Nov 7 '10 at 17:34
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How about \#? I think this is what you want.

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I think you're looking for \ast:

\documentclass{article}

\newcommand\connectedsum{\ensuremath{\,\ast_{i=1}^n}\,}

\begin{document}

    $F_i\connectedsum G_i$

\end{document}
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