Give me my fancy math 𝒳 from mathptmx back!

Question

In the old times of mathptmx, we wrote stuff such as

\documentclass{article}
\pagestyle{empty}
\usepackage{mathptmx}
\begin{document}
Let \(\mathcal{X}=\{X_1,\ldots,X_n\}\).
\end{document}

and got from pdflatex a fancy calligraphic math 𝒳, which was sufficiently fancy to be visibly distinct from the remaining italicized math 𝑋 in 𝑋₁ till 𝑋ₙ:

However, in the more modern times, mathptmx has been obsoleted in favor of newtx. Feeding

\documentclass{article}
\pagestyle{empty}
\usepackage{newtxmath}
\begin{document}
Let \(\mathcal{X}=\{X_1,\ldots,X_n\}\).
\end{document}

to pdflatex results in a dull

The distinction between the first X and the remaining occurrences is far less prominent. This is unfortunate, as it requires additional attention from the reader to distinguish between the two.

For pdflatex, what is the advised way of getting the fancy math 𝒳, such as in mathptmx, , or similar, if you use newtxtext and newtxmath for standard text and math fonts? Yes, I know that one can use an image (with the traditional caveats of, e.g., X not appearing in the PDF text layer), and, I know there is mathrsfs, which is not obsolete in CTAN but is pretty dated (1999-06-30 is the date of the file mathrsfs.sty on the CTAN file system and 1996-01-01 is the internal date stated inside mathrsfs.sty). Is it still advisable to use mathrsfs+\mathscr or are there better options?

Crosspost: http://latex.org/forum/viewtopic.php?p=112487 .

Answer 1

You can get the desired X by loading the mathrsfs ("Ralph Smith's Formal Script") font package and writing \mathscr{X}.

\documentclass{article}
\usepackage{newtxtext,newtxmath}
\usepackage{mathrsfs}
\begin{document}
Let \(\mathscr{X}=\{X_1,\ldots,X_n\}\).
\end{document}

Answer 2

The author of newtxmath also wrote (and actively maintains) the package mathalpha, to easily declare calligraphic, script, fraktur, and blackboard bold math alphabets. With mathalpha, you can get your script X with the following example.

\documentclass{article}
\usepackage{newtxmath}
\usepackage[cal=rsfs]{mathalpha}
\begin{document}
Let \(\mathcal{X}=\{X_1,\ldots,X_n\}\).
\end{document}

By using scr instead of cal, the X will be obtained from the command \mathscr instead of the command \mathcal.

Answer 3

mathptmx uses this font here:

\documentclass{article}
\pagestyle{empty}
\usepackage{newtxmath}
\DeclareMathAlphabet{\mathcal}{OMS}{ztmcm}{m}{n}

\begin{document}
Let \(\mathcal{X}=\{X_1,\ldots,X_n\}\).
\end{document}