How to write an encircled bullet?

Question

How can I write in MathJax a circle with a bullet inside it? I know \odot, but this is a circle with a dot inside it; I want most of the circle be filled by the dark circle inside and only a thin clear rim surrounding it.

Answer 1

In Mathjax, it is hard to overlay characters and get them perfectly aligned on all screens. ($\rm\LaTeX$ has purpose-built packages.) Instead, you can use the following Unicode character (which Mathjax allows):

⦿
CIRCLED BULLET
Unicode: U+29BF, UTF-8: E2 A6 BF

$$a⦿b. ⦿^{⦿^{⦿^{⦿}}}.\Huge ⦿.$$ gives $$a⦿b. ⦿^{⦿^{⦿^{⦿}}}.\Huge ⦿.$$ It even comes with the spacing of a binary operator, $a⦿b$ $a{⦿}b$$a⦿b$ $a{⦿}b$

Here's a screenshot of the above using two different renderers, HTML-CSS (with my preinstalled XITS font):

and common HTML:

If you don't want to copy-paste and have good memory, one can type \unicode{x29bf}$\unicode{x29bf}$ for the same effect. (This works for other Unicode symbols, e.g. \unicode{x1D7D9}$\unicode{x1D7D9}$. I am both lazy and forgetful, so I use software.)

• Some other circled Unicode symbols: $⊕⊖⊗⊘⊙⊚⊛⊜⊝⨀⨁⨂⨷⨸⦶⦷⦸⦹⦺⦻⦼⦽⦾⦿⧀⧁$.
• I couldn't find a big version to make the equivalent of $⨀\limits_a^b$$⨀\limits_a^b$, but Davide Cervone in the comments below has given the following beautiful suggestion (copied verbatim)

You might consider \mathop{\vcenter{\huge ⦿}}_{n=1}^{\infty} as more semantic and a little easier to use, plus it centers the operator, and \limits will work with that: $$\mathop{\vcenter{\huge ⦿}}_{n=1}^{\infty} abc$$ Even better might be \mathop{\vcenter{\huge\unicode[.67,.15]{x29BF}}}_{n=1}^{\infty}, which specifies the height and depth of the character so that the limits can be better placed (though this may depend on the fonts that the viewer has installed).