# How to see the \O in math-mode (at LyX)

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}

• You probably want \emptyset, not \O; or \varnothing from amssymb. – egreg Nov 14 '15 at 14:31
• sorry there is a \makeatletter hiding some lines above. i deleted previous comment. – David Carlisle Nov 14 '15 at 14:55
• 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
• 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
• @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

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