In math mode, ignores all spaces. It has complicated built-in rules which it uses to determine how the equation should be spaced. (For instance, it knows to put a little extra space around + and = signs.) But sometimes you want to put words in the middle of an equation. For instance, you might want to generate

The way you do this is to use the `\text{ }`

command to
tell to go back into text mode briefly. In our example,
you would type

$$ \{\psi \mid \psi \text{ is an additive character}\} $$.Then will set ``is an additive character'' with a space before it in text mode. Note that if we had left out the space before ``is'', we would have got

This brings up a point about using math mode properly. In math
mode, assumes that any string of letters next to one
another is a product, and spaces it accordingly. If, in fact,
it was a word, it won't look good. So, for symbols which are
actually parts of words, like Stab, Spec, Proj, *etc.*, you should
use `\operatorname{ }`

to tell to space the letters in the
symbol as text, not as a product. For instance, type

$$ X := \operatorname{Spec}A $$to get

You can make subscripts and superscripts of symbols you have generated
using the `\operatorname{ }`

command: e.g.,
`$\operatorname{Stab}_G(H)$`

gives .