Is it possible to typeset this double-X symbol?

Question

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?

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 07 '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: https://books.google.de/books?id=BM1ckQKCXP8C&printsec=frontcover&dq=local%20polynomial%20modelling%20and%20its%20applications%20theorem%203.1&hl=en&sa=X&ved=0ahUKEwiagoCe0vfXAhVJI1AKHVEeAqUQ6AEIKTAA#v=snippet&q=the%20following%20bias-variance&f=false — Epiousios, Dec 07 '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, Dec 07 '17 at 23:03

Werner · Accepted Answer · 2017-12-07T17:08:08.900

8

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}

edited Dec 07 '17 at 17:08

answered Dec 07 '17 at 16:33

Werner

603,163

Funny. Just after I commented, that one should use math choices your answer pops up :) – Skillmon Dec 07 '17 at 16:34
You should add an additional grouping around the definition of \XX for super- and subscripts. – Skillmon Dec 07 '17 at 16:46

egreg · Answer 2 · 2017-12-07T16:49:28.823

5

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.

edited Dec 07 '17 at 16:49

answered Dec 07 '17 at 16:38

egreg

1,121,712

Why \mspace instead of \mkern? – Andrew Swann Dec 07 '17 at 17:05
@AndrewSwann Because it has a more LaTeX-like syntax. – egreg Dec 07 '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 07 '17 at 17:08
@Manuel The mu is certainly needed. – egreg Dec 07 '17 at 17:09
@egreg Oh, yes, I mixed concepts. – Manuel Dec 07 '17 at 17:31

Is it possible to typeset this double-X symbol?

2 Answers2