How to add a font style for a specific letter only?

Question

In my latex document I of course have the normal lower case letter "k" in the default font. What I want to do is add the script version of this, "𝓀", to my document. I am not sure if I need to find a new font type so I can have $\mathscr{k}$ return 𝓀 (or some other math font command like mathpzc or mathcal), or if I need to find a package that will add the 𝓀 as a separate character. In either case, I don't want to change anything else about my document. I only want to change $\mathscr{k}$ to give 𝓀 or add the character itself. I have no idea where to start.

Answer 1

You can borrow the math script alphabet from STIX2.

\documentclass{article}

\DeclareFontEncoding{LS1}{}{}
\DeclareFontSubstitution{LS1}{stix2}{m}{n}
\DeclareMathAlphabet{\mathscr}{LS1}{stix2scr}{m}{n}

\begin{document}

$\mathscr{k}k$ and also $\mathscr{A}+\mathscr{V}$

\end{document}

Answer 2

Several packages allow you to use \mathscr{k} in math mode. I recommend using unicode-math in LuaLaTeX when you can, and legacy fonts when you have to.

Here is a MWE that sets up the Libertinus fonts through fontsetup, which loads unicode-math.

\documentclass{article}
\usepackage[libertinus]{fontsetup}

\pagestyle{empty}

\begin{document}

$\mathscr{k}k$ and also $\mathscr{A}+\mathscr{V}$

\end{document}

Any OpenType math font will support lowercase script letters, although some might not use the style you prefer. It is possible to substitute another font’s \mathscr alphabet with the range= option of \setmathfont. This MWE substitutes the alternative \mathscr alphabet of Stylistic Set 1 from STIX Two Math, which you will notice is less-heavily slanted.

\documentclass{article}
\usepackage[libertinus]{fontsetup}

\setmathfont{STIX Two Math}[
  range={scr,bfscr},
  Scale=MatchUppercase,
  StylisticSet=1 ]

\pagestyle{empty}

\begin{document}

$\mathscr{k}k$ and also $\mathscr{A}+\mathscr{V}$

\end{document}

In PDFLaTeX, you can select any script alphabet that supports lowercase letters with mathalpha, for example \usepackage[scr=boondoxo]{mathalpha}.

\documentclass{article}
\usepackage[scr=boondoxo]{mathalpha}

\pagestyle{empty}

\begin{document}

$\mathscr{k}k$ and also $\mathscr{A}+\mathscr{V}$

\end{document}