How do I create a major arc open to the left?
Here is a solution that uses \ooalign
to overlay a white rectangle on part of a \medcirc
.
\mathchoice
is used to allow your symbol to appear in script or scriptscript styles. You can edit the \rule
arguments to adjust height and thickness of the opening in each style.
\documentclass{amsart}
\usepackage{txfonts,color}
\newcommand{\arcl}{\mathchoice%
{\mathbin{\ooalign{\hfil$\medcirc$\hfil\cr\textcolor{white}{\rule[.25ex]{.2em}{.65ex}}\hfil\cr}}}
{\mathbin{\ooalign{\hfil$\medcirc$\hfil\cr\textcolor{white}{\rule[.25ex]{.2em}{.65ex}}\hfil\cr}}}
{\mathbin{\ooalign{\hfil$\scriptstyle\medcirc$\hfil\cr\textcolor{white}{\rule[.2ex]{.2em}{.4ex}}\hfil\cr}}}
{\mathbin{\ooalign{\hfil$\scriptscriptstyle\medcirc$\hfil\cr\textcolor{white}{\rule[.15ex]{.2em}{.25ex}}\hfil\cr}}}
}
\begin{document}
$a\medcirc b_{a\medcirc b_{a\medcirc b}}$
$a\arcl b_{a\arcl b_{a\arcl b}}$
\end{document}