# Is it possible to typeset this double-X symbol?

I'm looking for this double-X symbol. Is this available in some package? If not, would it somehow be possible to recreate this symbol by creating a new symbol which consists of two Xs printed on top of each other?

• Are you aware of this post? – user121799 Dec 7 '17 at 16:24
• Thank you, detexify did not find anything unfortunately.. – Epiousios Dec 7 '17 at 16:29
• welcome to tex.sx. this reminds me of the "smash product" (unicode U+2A33, \smashtimes in unicode-math), but i think it isn't meant to be that. and i don't remember seeing it in any of the "usual" symbol fonts. can you provide more context -- source, meaning, ... -- if it's a "recognized" symbol, it can be requested from unicode if proper documentation is provded. (it's easy enough to construct, but i'd prefer some clarification first.) – barbara beeton Dec 7 '17 at 16:46
• @barbarabeeton It's an abbreviation for the vector (X_1,...,X_n) and it's introduced on page 14 of this book: books.google.de/… – Epiousios Dec 7 '17 at 16:51
• Note that perhaps that is intended as a boldface X, but was typed in such way because of a lack of a better option. – Pedro Tamaroff Dec 7 '17 at 23:03

You should define a macro for that symbol, say \XX that inserts a negative space between the two X's. This space should depend on where it's used inside math mode (using \mathchoice):

\documentclass{article}

\usepackage{amsmath}

\newcommand{\XX}{% Space between XX varies depending on where it's used
\mathchoice% https://tex.stackexchange.com/q/148740/5764
{X\mskip-12.5mu X}% \displaystyle
{X\mskip-12.5mu X}% \textstyle
{X\mskip-12mu X}% \scriptstyle
{X\mskip-11.5mu X}% \scriptscriptstyle
}

\begin{document}

$\text{Var} \bigl( \hat{\beta} \,\vert\, \XX \bigr) \quad \XX^{\XX^{\XX}}$

\end{document}

• Funny. Just after I commented, that one should use math choices your answer pops up :) – Skillmon likes topanswers.xyz Dec 7 '17 at 16:34
• You should add an additional grouping around the definition of \XX for super- and subscripts. – Skillmon likes topanswers.xyz Dec 7 '17 at 16:46

The correct spacing here is to be inserted with \mspace, so it automatically works also in subscripts and superscripts.

\documentclass{article}
\usepackage{amsmath}

\DeclareMathOperator{\Var}{Var}

\newcommand{\doubleX}{{%
X%
\mathchoice{\mspace{-12.5mu}}{\mspace{-12.5mu}}{\mspace{-12mu}}{\mspace{-11mu}}%
X%
}}

\begin{document}

$\Var(\hat{\beta} \mid \doubleX) \, \doubleX^{\doubleX^\doubleX}_{\doubleX}$

\end{document}


Different choices have to be made for the different styles; the values may change if you use a different font.

• Why \mspace instead of \mkern? – Andrew Swann Dec 7 '17 at 17:05
• @AndrewSwann Because it has a more LaTeX-like syntax. – egreg Dec 7 '17 at 17:07
• In this case the mu is not needed (not bad, but not the key part) because of \mathchoice, right? – Manuel Dec 7 '17 at 17:08
• @Manuel The mu is certainly needed. – egreg Dec 7 '17 at 17:09
• @egreg Oh, yes, I mixed concepts. – Manuel Dec 7 '17 at 17:31