# Double struck 1 in Mathjax

I want to type the 𝟙 symbol in Mathjax (physics.stackexchange) without having to copy the Ascii character. I found https://tex.stackexchange.com/questions/204998/double-struck-zero-and-one which mentioned

 \mathbbm{1}
\mathds{1}
\mathbb{1}


The first two options don't work and the last option looks like a regular 1. Since it's on stackexchange I can't import modules as far as I know.

• $\mathbb{1}$ seems to do the job: $\mathbb{1}$. That being said, MathJax is not TeX, and does not support all of the features that a real TeX or LaTeX implementation supports. – Xander Henderson Mar 14 at 23:55
• @XanderHenderson in a previous meta post link (possible dupe?) I found out that this is browser/math renderer dependent. It works on Macs and iOS devices – Calvin Khor Mar 15 at 1:20
• @CalvinKhor Good to know. It works for me. That being said, I'll reiterate my previous comment: MathJax is not TeX. :\ – Xander Henderson Mar 15 at 1:22
• Indeed, how I wish TeX allowed me to mathbb my 1s without weird packages – Calvin Khor Mar 15 at 1:24
• @CalvinKhor The dsfont package isn't that big a deal. I wanted a doublestruck / blackboard bold $i$ for my thesis, and had to engage in all kinds of hackery to get that to work. – Xander Henderson Mar 15 at 2:20
• @TeresaLisbon as in the linked post you can also memorise x1D7D9 to write \unicode{x1D7D9}$\unicode{x1D7D9}$ which can be wrapped in a newcommand – Calvin Khor Mar 15 at 7:13
• @CalvinKhor I see! Will use that in the future. – Teresa Lisbon Mar 15 at 7:26
• I find that $\mathbb{1}$ works only if I change the Math Renderer to MathML. (Firefox 86.0.1 on Windows 7.) – Peter Phipps Mar 15 at 8:25

This is nearly a dupe but seeing how no one voted to close maybe I can try to summarise the information better than the exploratory post there. AFAIK the current options are

Description Input Output
Copy paste the Unicode symbol 𝟙 $𝟙$ $$𝟙$$
The naive \mathbb1 (not reccomended: only works with some setups e.g. HTML-CSS renderer on machines with the STIX fonts installed) $\mathbb1$ or $\Bbb1$ $$\mathbb1$$ or $$\Bbb1$$
Memorise the hexadecimal number that encodes 𝟙, x1D7D9 (the Ds are case insensitive) $\unicode{x1D7D9}$ $$\unicode{x1d7d9}$$

A possible alternative to using \newcommand would be to use some sort of clipboard feature. (This may involve installing software.) For instance Macs come with a system wide text replacement feature built-in; so one can make something like mseone automatically turn into \unicode{x1d7d9}; some details are in this answer.

The following 'substitutes' are easier to use:

Input Output
$\mathbf1$or $\bf1$ $$\mathbf1$$ or $$\bf1$$
$\pmb1$ or $1\!\!1$ $$\pmb1$$ or $$1\!\!1$$
$1\!\rm l$ (variant of the above) $$1\!\rm l$$
• One can get creative e.g. \pmb can be iterated: $$\pmb{\pmb{\pmb{\pmb{1}}}}$$ but if you're going to do something hard, you might as well use \unicode{x1D7d9}.

• Since $$1+2=3$$ and x1D7D9+2=x1D7DB, the code $\unicode{x1D7DB}$ outputs $$\unicode{x1D7DB}$$.

• Having \mathbb1 be equivalent to the Unicode input \unicode{x1d7d9} is a planned feature of Mathjax v3, but (IIRC) at the time of writing Mathematics SE does not have plans to use Mathjax v3.

• Perusing a table of characters, here is a non-exhaustive list of other mathematical Unicode symbols that I do not know the equivalent Mathjax code for that nonetheless display well for me in Safari v14.0.3 on my Mac (but some have obvious substitutes):

Description Output
variant integral signs $$∯∰⨕∱∲∳⨋⨍⨎⨏⨖⨗⨘⨙⨚⨛⨜$$
variant equality symbols $$≉≋≌≎≏≐≑≒≓≔≕≖≗≘≙≚≛≜≝≞≟≣⊜$$
variant binary operators/relations $$⊌⊍⊎⊑⊒⊏⊐⊓⊔⊘⊚⊛⊝⊞⊟⊠⊡⨃⨄⨴⨵⨶⨷⨸⨹⨺⨻$$
degree symbol 5° (distinct from ^\circ$$^\circ$$ or ^o$$^o$$) $$5°$$
greek or lowercase in blackboard-bold $$ℿ⅀ℽ𝕒𝕓𝕔𝕕𝕖𝕗𝕘𝕙𝕚𝕛𝕜𝕝𝕞𝕟𝕠𝕡𝕢𝕣𝕤𝕥𝕦𝕧𝕨𝕩𝕪𝕫$$

(not included in the list: \iiiint, \oint $$\iiiint,\oint$$, \cong, \simeq, \equiv$$\cong,\simeq,\equiv$$, \otimes, \oplus, \ominus, \odot, \bigoplus, \bigotimes, \bigodot $$\otimes,\oplus,\ominus,\odot,\bigoplus ,\bigotimes,\bigodot$$.) Some of them need extra code; consider ⨃_{a\in\Lambda} U_a vs ⨃\limits_{a\in\Lambda} U_a $$⨃_{a\in\Lambda} U_a \quad \text{vs}\quad ⨃\limits_{a\in\Lambda} U_a$$

• Thanks for this extensive answer! The \mathbb 1 renders like a regular one for me so I guess I'll have to stick to copying the Unicode symbol. – AccidentalTaylorExpansion Mar 15 at 9:36
• @AccidentalTaylorExpansion you don't like memorising x1D7D9? :) I've certainly memorised it after writing this answer... You're welcome – Calvin Khor Mar 15 at 9:51
• @AccidentalTaylorExpansion I just thought of another way: the chief problem is the ability to turn something you type into something that displays as $\unicode{x1d7d9}$. You can achieve this with a dedicated clipboard program that can store snippets of text (like \unicode{x1d7d9}) and have them paste this in your text field when you type a keyword. I’ve added this to the answer (but unfortunately I don’t know of specific programs for this outside of a built in feature of Mac OS; ‘there must be an app for that’. Suggestions welcome) – Calvin Khor Mar 16 at 4:58