Including letters from other fonts

Question

Nowadays, book printers mix different fonts. Aa example of this is Springer, which writes in Times New Roman but takes letters g, v and greek psi (I don't know if more) from computer modern (i think).

How can I do the same? Hoe can import a single letter from another family font?

The closest question I have found is Unicode selection from other font but I don't know understand anything, and import symbols from other packages seems me not be valid for this case.

SOME EXAMPLES:

from Hussemoller's Fibre bundles book.

from A. O. Barut, R. Raczka Theory of group representations and applications, Polish Scientific Publishers distribution by Ars Polona (1980).

EDIT.

Talking yesterday with @cfr I notice I must make two changes:

1.- Certainly, I was thinking about use these import glyphs only in math mode. However, thinking better later, I realize that Springer also uses the ''imported'' glyphs in theorem-like environments (see the 'v' oth Theorem 3.2).

2.- When I post this question I was sure they import glyphs from another family fonts (some people in the typography.guru forum confirmed me that). However, some days later I discovered this PDF, shows that, at least in the case of Times New Roman, there was extra glyphs similar to other fonts. I haven't found the same for Barut's book yet, but it seesm me reasonable to think something similar happens and they don't import extra characters, but they font has extra glyphs.

Answer 1

In the modern toolchain with unicode-math, you can do this in math mode with \setmathfont[range= ,,,, For example:

\documentclass[varwidth]{standalone}
\usepackage{unicode-math}

\defaultfontfeatures{Scale = MatchLowercase}
\setmainfont{TeX Gyre Termes}
\setmathfont{TeX Gyre Termes Math}
\setmathfont[range={\mitv,\mitnu}]{XITS Math}

\begin{document}
Text mode: \textit{vν}.

Math mode: \( \symit{vν} \).
\end{document}

For a bit of explanation, the \mit* macros are Unicode characters from the mathematical alphanumeric code blocks defined in Unicode, and \symit explicitly selects mathematical italic symbols. You could also write \mitv\mitnu.

That this does not work if you type $v$ should probably be considered a bug, or at least a limitation, in unicode-math (as of v0.8n).

Some OpenType fonts select alternative shapes for letters such as g, v, w and z with character variants, stylistic sets or alternatives. For example, Stix Two Math changes from a rounded v to a pointed v that’s more similar to the Greek letter ν if you select its stylistic set 3.

In text mode, you can declare a \newfontfamily using fontspec and switch to it. For example: \DeclareRobustCommand\textnu{{\greekfamily ^^^^03bd}}. The newunicodechar and ucharclasses packages might help if you want to be able to type in ν and have it work automatically. For example, \newunicodechar{ν}{\textnu}.

In a legacy NFSS document, compatible with PDFTeX, you could declare a new math alphabet and add new commands (You might name them something like \varv and \varnu, or \mathalt{v}) that load from that math alphabet. You can also \renewcommand\nu. Some font packages, notably newtxmath, give you package options to replace particular glyphs. To hand-roll this yourself so that you can just write $v$ in your document would take more work, but you’d probably want a virtual font.

Answer 2

It is a partial satisfactory answer.

The solution to get something similar to Husemoler's book it is enoguh to consider the varg option in the newtxmath package. It is a partial solution by two reasons:

1.- Because the italic ''z'' isn't like ''z'', but i has... a curve? I don't know the exact term, bu you know.

2.- Binary and relation symbols like '\subset', '\cap' '\in'... are different. In fact, these springer symbols are closer to the matha otion of the mathabx package than others.

In any case, springer books use Times 4-line Mathematics Series 569 and some variants of that.

Regarding Barut's book, I can't say anything unfortunately.

More avdanced and complete answers are welcome.