# How to typeset an open circle with a dot inside, on an Internet forum?

I need to somehow put this symbol:

http://wstaw.org/m/2014/12/20/cauchy_cropped_cropped.png

And of course I mean this semi-open circle with a dot inside, right after "a_n".

I have seen it in a TeX document, this one: http://www.mimuw.edu.pl/~mmoszyns/Analiza-dla-informatykow-2014-15/analinf127.pdf (for example on page 48/49), where it is used to denote the Cauchy product.

I could not find neither this symbol nor a dotless open circle in The Comphehensive LaTeX symbol list (http:// www.tex.ac.uk/ctan/info/symbols/comprehensive/symbols-a4.pdf).

To make things even more awkward, I need to put it on an Internet forum. Due to that this idea \odot \!\! \white{\bullet} will not work: the background is not white, and even worse, it changes from post to post.

Thanks in advance for any idea how to do this.

EDIT: I do not want a normal, "closed" circle such as \odot or \bigodot, but an "open" one, as in the image above. To be more specific: I want this: http://wstaw.org/m/2014/12/22/ccccc.png and not this: http://wstaw.org/m/2014/12/22/6718f3851aba6f5727da3da31bc74ee2.png

Also, since this is for an Internet forum, I can't \def anything.

• the symbol is \bigodot, and it is in the comprehensive symbols list (p.25 in the lettersize version). it's unicode U+2A00. as for putting it on an internet forum, better to trust someone else. – barbara beeton Dec 22 '14 at 14:36
• the packages latexsym, amsfonts, amssymb, txfonts, pxfonts, and wasysym define\odot for that. – user10274 Dec 22 '14 at 14:38
• No and no. \bigodot and \odot both define a "normal", closed circle - and I want an open circle. See the attached picture. – gaazkam Dec 22 '14 at 14:40
• On page 2 of the PDF, you find the authors email-address. Why not send him a friendly note and ask for it. Do you have some other reference for this symbol. Should that be part of the unicode? (can't find it there, though) The author writes "(raczej niespotykanym gdzie indziej...)/ rather unusual elsewhere ". Maybe you should search for a symbol, that your readers already know. – LaRiFaRi Dec 22 '14 at 15:05
• This guy is my lecturer... and the document is the script for my classes... And he didn't do the TeXing himself, but instead asked those people noted at the beginning of page 2 to do it, and their e-mail adress is not given there. I can find them through USOSweb, though. – gaazkam Dec 22 '14 at 15:13

## 2 Answers

EDITED to use \clipbox instead of white \rule overlay.

\documentclass{article}
\usepackage{amssymb,xcolor,trimclip}
\def\opendot{\mathbin{\clipbox{0pt 0pt .55ex 0pt}{$\odot$}}}
\begin{document}
$\sum_{n=0}^{+\infty} a_n \opendot \sum_{n=0}^{+\infty} b_n := \sum_{n=0}^{+\infty} c_n$
\end{document}


ADDENDUM:

Apparently, the web-based input available to the OP did not allow for the clipped solution. However, I suggested (in the comments) something akin to {{\textsf{C}\kern-.85ex\raisebox{.9pt}{$\cdot$}\kern .25ex} which sufficed for the needs of the OP.

\documentclass{article}
\begin{document}
$x\mathbin{\textsf{C}\kern-.85ex\raisebox{.9pt}{$\cdot$}\kern .25ex}y$
\end{document}


• I guess this symbol is meant to be a binary operator acting on both sums, so \mathbin instead of \mathop might be "more" correct. – Henri Menke Dec 22 '14 at 14:55
• @gaazkam If I was really in a bind, perhaps {\textsf{c}\kern-.7ex\raisebox{-.25pt}{$\cdot$}} – Steven B. Segletes Dec 22 '14 at 16:20
• @gaazkam If your question is about MathJax, then it's off topic here. With MathJax you can only use symbols that are in Unicode. – egreg Dec 22 '14 at 16:37
• @gaazkam You could use a capital "C" instead of a lowercased "c". As in {\textsf{C}\kern-.85ex\raisebox{.9pt}{$\cdot$}\kern .25ex} – Steven B. Segletes Dec 22 '14 at 16:41
• I wish I could upvote twice, once for the answer and once for the dialogue with the OP. – Ethan Bolker Dec 22 '14 at 17:04

Another solution (credit goes to a4karo from matematyka.pl):

A \tikz{\draw (30:.3em) arc [start angle=30,end angle=330,radius=.3em]; \draw (0,0) circle (.1pt);} B=C

http://wstaw.org/m/2014/12/23/9ff78e6d6784034c94d199ecb79ca1f7.png

$$A \tikz{\draw (45:.3em) arc [start angle=45,end angle=315,radius=.3em]; \fill (0,0) circle (.8pt);} B=C$$

http://wstaw.org/m/2014/12/23/dbf40b061d7af9e78eed82589e13d4cb.png

The parameters need to be manually adjusted depending on context, though:

http://wstaw.org/m/2014/12/23/7ae6b261cda0139def999a78900031cf.png

http://wstaw.org/m/2014/12/23/1ebab21212431359471f25a9b73d2107.png

But I still think this is a worthy alternative.