如何在数学模式中查看 \O（在 LyX）

Question

我正在使用 LyX 2.1.2，当我输入（数学模式）以下内容时：\O我在 pdf 文件中看不到任何内容。
你知道为什么吗？

如果这有帮助 - 当我查看源代码时 - 这是我看到的：
\L{$\O$}
但是当我把它放在 ERT 时：$\O$没有问题，我在 pdf 文件中看到它......

谢谢你！

以下是该文件的源代码：

%% 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}

Answer 1

您可能想要\emptyset，而不是\O；或者\varnothing来自amssymb。