Q:

mathematical symbols

I'm using multidimensional formulas in my work and to differ it from the onedimensional ones I denote it by \mathbf, but what am I supposed to use in the definition and theorems environment to look samely almost like italic?

A:

It's usually a bad idea to make the font used for mathematical symbols depend on the text font used for the surrounding environment. Thus if you are using upright bold for the symbol then best to do so even if your theorem text is italic.
Or you may prefer to use bold math italic instead of \mathbf eg using \bm{A} from the bm package, but use the same version consistently, that is don't switch between \bm and \mathbf depending on the surrounding text font.

