Please note, you are not answering my question. I said: when we need italicized and bold, none of bf, textb or mathbf combined with mathit produce it. Up to what I know, only textbf & textit does it. Do you know an alternative?
So if you have a way to produce italicized and bold at the same time, please say it. Otherwise the code remains with textbf & textit when it needs to address that particular requirement until we discover something better.
For an example, easy: open www.overleaf.com, where you can try for free, fast and with ease, and input
And you will see italicized and bold. Try now changing texit by mathit and feel free to change textbf for whatever you want, click on Recompile to see the output. If you get italicized and bold, please post here what combination of mathit with whatever you used, and I will think about again.
The example, anyway, is not as artificial as you may think, consider this real case scenario:
> EllipticK(z); # with Typesetting = extended and SpecialFunctionRules ON, it is a K in bold & italics
For the notational conventions used to display this or other mathematical functions that also use bold and italicized, see the NIST - Digital Library of Mathematical Functions which is the main reference nowadays.
Edgardo S. Cheb-Terrab
Physics, Differential Equations and Mathematical Functions, Maplesoft