# How to write character x in math enviroment as Polykov's book?

I am using Latex. I want to write x as in Polykov's book, but I do not konw which font it is.

It is like chi, but it is not.

## 2 Answers

It seems to be Bitstream Charter text with Math Design math.

It is just an x, not chi.

Use \usepackge[charter]{mathdesign} to employ this type of font.

More information about free fonts in Latex, see enter link description here

• Thanks. I will try this package. – thone May 11 '20 at 5:09
• If I use this package, then all the characters are changed. Is there any way just to change the character which I want to make different? for example, like \mathcal{}. – thone May 11 '20 at 8:43

The xcharter package has five different recommended preambles for this font, all of them for PDFLaTeX. The disadvantage of using mathdesign is that it breaks mathalpha, making it difficult to replace the default \mathcal with a better one.

If you’re using the modern toolchain, you would load the letters of XCharter over your math font, for example:

\documentclass{book}
\usepackage[math-style=ISO]{unicode-math}

\defaultfontfeatures{Scale=MatchLowercase}
\setmainfont{XCharter}[Scale=1.0]
\setmathfont{Libertinus Math}
\setmathfont[range=it]{XCharter Italic}
\setmathfont[range=up]{XCharter Roman}
\setmathfont[range=bfit]{XCharter Bold Italic}
\setmathfont[range=bfup]{XCharter Bold}

\newcommand\dint[1]{\mathop{\mathrm{d}{#1}}}

\begin{document}
$G \left( x, x' \right) = \int \left( \frac{\mathcal{D} x(\tau)}{\mathcal{D} f(\tau)} \right) \exp \left( -m_0 \int\limits_0^1 \left(\dot{x}^2 (\tau)\right)^{1/2} \dint{\tau} \right)$
\end{document}


XCharter is a fork of the free version of Charter that Bitstream donated to the X Consortium in 1992. One disadvantage is that v and ν (nu) might be too similar. Charis SIL is another free font based on Charter.

(Thanks to Sebastiano for some tips on better matching the typesetting of the book. You might try \frac{1}{2} or \sfrac from xfrac instead of {1/2}.)

• I tried the package, but it does not work. Maybe my latex is too old. – thone May 13 '20 at 12:42