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