[texhax] mathfont

[Robert Rogers]

Good morning! -- This is Robert Rogers hailing.

How does one change the font displayed by default in math mode?
That is, text bracketed by dollar signs?  For example, $x^{2}$?

I am wondering if there is a generic macro
or perhaps a command kin to 

\renewcommand{\rmdefault}{ppm}

where one can reassign the underlying default.
Is that default \mathfam or something of that nature?

Thanks for your help! -- Robert