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How can I typeset the following expression?

enter image description here

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1  
    
@AlexNelson i have :undefined control sequence $\displaymode –  researcher Apr 18 at 20:58
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Heh, I'm working on Java while answering a TeX question, I mean $\displaystyle\bigl\{O^{(i)}, e^{(i)}\bigr\}^{M}_{i=1}$ –  Alex Nelson Apr 18 at 21:02
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@researcher The best answer should probably also mention Knuth notes that squiggly brackets should have some small spacing, \bigl\{\,..., ...\,\bigr\}. Also, using \bigl\{ and \bigr\} works great...for inline or display stuff. For footnotes, I think you should use \left\{...\right\}. –  Alex Nelson Apr 18 at 21:05
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@AlexNelson \displaystyle is useless. –  egreg Apr 18 at 21:16

2 Answers 2

up vote 8 down vote accepted

This code should give you what you want:

\documentclass{article}
\usepackage{mathtools}

\begin{document}
\begin{equation*}
\left\{\mathrm{O}^{\left(i\right)},\mathrm{e}^{\left(i\right)}\right\}_{i=1}^M
\end{equation*}
\end{document}

enter image description here

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7  
I hope you don't mind, I've completed your code to give an MWE. –  LaRiFaRi Apr 19 at 0:44
2  
My guess is that O and e are bold: \mathbf{O}^{(i)} and \mathbf{e}^{(i)} (no need for \left and \right here). –  Heiko Oberdiek Apr 19 at 2:26

To fully reproduce the screenshot you've posted, you need (as also pointed out in a separate comment by Heiko Oberdiek):

  • \left\{ and \right\} for auto-scaled curly braces

  • \mathbf{O} and \mathbf{e} to typeset "O" and "e" in math-bold

A full MWE (minimum working example):

enter image description here

\documentclass{article}
\begin{document}
\[
\left\{ \mathbf{O}^{(i)},\mathbf{e}^{(i)} \right\}_{i=1}^M
\]
\end{document}
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