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I'm using LyX 2.1.2 and when I put (at math-mode) this: \O I see nothing at the pdf file.
Do you know why?

If this helps - when I look at the source code - this what I see:
\L{$\O$}
But when I put it at ERT: $\O$ there is no problem and I see it at the pdf file....

Thank you!

Here is the source code of the file:

%% LyX 2.1.2 created this file.  For more info, see http://www.lyx.org/.
%% Do not edit unless you really know what you are doing.
\documentclass[english,hebrew]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\setlength{\parindent}{0bp}

\makeatletter
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Textclass specific LaTeX commands.
\usepackage{theorem}
\theorembodyfont{\upshape}
\newtheorem{theorem}{\R{משפט}}[section]
\AtBeginDocument{\make@lr\thetheorem}

% The following chunk fixes export with XeTeX.
% It is needed because polyglossia is used by default
% and \make@lr is only defined by babel.
\@ifundefined{make@lr}
{\def\make@lr#1{\begingroup
    \toks@=\expandafter{#1}%
    \edef\x{\endgroup
  \def\noexpand#1{\noexpand\@number{\the\toks@}}}%
  \x}}{\relax}

\makeatother

\usepackage{babel}
\begin{document}
\L{$\O$} -

$\O$
\end{document}
  • 1
    You probably want \emptyset, not \O; or \varnothing from amssymb. – egreg Nov 14 '15 at 14:31
  • 1
    sorry there is a \makeatletter hiding some lines above. i deleted previous comment. – David Carlisle Nov 14 '15 at 14:55
  • 2
    using \O for the empty set is wrong, it is the text letter O with stroke, any similarity with emptyset is accidental. you probably want \varnothing – David Carlisle Nov 14 '15 at 15:05
  • 1
    Actually not accidentally, the symbol for the empty set originated from the Scandinavian ø (the originator was French, I think, and he knew some Norwegian). But nowadays, especially in latex, special math symbols are used, not the \O as it is a text only symbol. – daleif Nov 14 '15 at 15:49
  • 1
    @DavidCarlisle Barbara recently emailed me in relation to some unicode about empty set and ø. I then asked a math historian. He even sent me a scan of on of the first (presumably) printed book that was using the symbol. It was clearly a lower case ø. I think the book was from 1939. – daleif Nov 14 '15 at 21:24
3

You probably want \emptyset, not \O; or \varnothing from amssymb.

| improve this answer | |
  • As you write me at the comments to my qestion ("using \O for the empty set is wrong, it is the text letter O with stroke, any similarity with emptyset is accidental."), \O is not a symbol of the empty set at all, so yes, I will use \emptyset or \varnothing for the empty set :-) – heblyx Nov 14 '15 at 15:18
  • @Yoar: Insert > Math > Inline Math > Miscellaneous > emptyset. – Werner Nov 14 '15 at 21:17
  • @Werner, where? I don't find it.... I saw: Insert > Math > Inline Math (at LyX), But I didn't found the next levels.... – heblyx Nov 14 '15 at 22:00
  • @Yoar: Miscellaneous is part of the math object menu. Here are some images to light the way: Insert > Math > Inline Math > Miscellaneous > emptyset – Werner Nov 14 '15 at 22:57
  • Actually the letter Ø is indeed the symbol. Check it here: jeff560.tripod.com/set.html look for "The null set symbol." – Paolo.Bolzoni Sep 13 '17 at 15:59

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