答案1
答案2
我会选择一种特殊的符号来传达这个想法:
\documentclass[twocolumn]{article}
\usepackage{amsmath,amssymb}
\DeclareRobustCommand{\upcomp}{{{-}\!\!{\bullet}}}
\DeclareRobustCommand{\downcomp}{{{\bullet}\!\!{-}}}
\newcommand{\RR}{\mathbb{R}}
\newcommand{\RRup}{\RR_{\upcomp}}
\newcommand{\RRdown}{\RR_{\downcomp}}
\begin{document}
We define $\RRup=\RR\cup\{+\infty\}$ and $\RRdown=\RR\cup\{-\infty\}$
with the topology induced by the usual extended real line.
Now we can use $C_0(\RRup)$ that takes less space.
\end{document}
替代\mapsfromchar
方案stmaryrd
:
\documentclass[twocolumn]{article}
\usepackage{amsmath,amssymb,stmaryrd}
\DeclareRobustCommand{\upcomp}{{-\mapsfromchar}}
\DeclareRobustCommand{\downcomp}{{\mapstochar-}}
\newcommand{\RR}{\mathbb{R}}
\newcommand{\RRup}{\RR_{\upcomp}}
\newcommand{\RRdown}{\RR_{\downcomp}}
\begin{document}
We define $\RRup=\RR\cup\{+\infty\}$ and $\RRdown=\RR\cup\{-\infty\}$
with the topology induced by the usual extended real line.
Now we can use $C_0(\RRup)$ that takes less space.
\end{document}