LaTeX math mode and mbox mode

Question

According to a doc I found around

An \mbox within math mode does not use the current math font; rather it uses the typeface of the surrounding running text.

In math mode, I would like to write something like a_{\mbox{foo}}. If I use this, the foo will be quite big, too big. If I write a_{foo}, foo will be in italic.

What is the magic trick to have non-italic, small text?

Answer 1

I personally prefer to use the \text{} command provided by the AMS-LaTeX package. To use this, you need to include the statement

\usepackage{amsmath}

somewhere in your document preamble, and then in any mathematical environment,

a_{\text{foo}}

will produce the desired output. Section 6 of the User’s Guide for the amsmath Package mentions that the \mbox{} equivalent is

a_{\mbox{\scriptsize foo}}

A final option is

a_{\mathrm{foo}}

which is what I used before I discovered AMS-LaTeX and the \text{} command.