Using long variable names in math mode in LaTeX

[Charles Payne 5542,16/119,x73770]

In Lamport's book, p. 53, he illustrates differences (sorry for the pun)
between typesetting in math mode and in text mode. I often want to use
long variable names in math mode, and I have discovered a special trick to
make the long variable name format as it would in ordinary text mode.

Under the old (pre-NFSS) LaTeX, I could do the following
${\em different}_n$
and LaTeX didn't complain. However, LaTeX with NFSS does. It says that
I cannot use \em, \bf, etc. in math mode.

Does anyone know how I code the expression above under LaTeX/NFSS to avoid
the typesetting woes illustrated on p. 53 of Lamport's book?

Thanks for your help.

Charlie