Most of the fonts used within math mode do not need to be explicitly invoked; but to use letters from a range of fonts, the following class of commands is provided.
\mathrm {<letters>}
\mathnormal {<letters>}
\mathcal {<letters>}
\mathbf {<letters>}
\mathsf {<letters>}
\mathtt {<letters>}
\mathit {<letters>}
These are also one-argument commands which take as an argument the
letters which are to be typeset in the particular font. The argument
is processed in math mode so spaces within it will be ignored. Only
letters, digits and accents have their font changed, for example
$\mathbf{\tilde A \times 1}$ produces 
.