How do you find the domain of a function without graphing it?
Real number symbol (UTF-8: ℝ) in XeTeX
calculus - Finding a function's domain from the function's formula - Mathematics Stack Exchange
How to write the domain and range of a function?
Videos
set of "input" or argument values for which a function is defined
I've been looking at all sorts of tutorials and walkthroughs on youtube and math-online, but I really can't get it
Please explain to me like I'm the idiot I am :)
How about this?
\documentclass{article}
\usepackage{amssymb}
\begin{document}
\begin{tabular}{ll}
Plain-\TeX{}: & ${\rm I\!R}$\\
amssymb: & $\mathbb{R}$
\end{tabular}
\end{document}
You should put your symbol format definitions in another TeX file; publications tend to have their own styles, and some may use bold Roman for fields like R instead of blackboard bold. You can swap nams.tex with aom.tex. I know, this is more common with LaTeX, but the principle still applies.
For example:
% paper.tex
\input nams.tex
$\realnumbers$ is connected.
% nams.tex
\def\realnumbers{\mathbb{R}}
% more definitions for the Notices.
% aom.tex
\def\realnumbers{\mathbf{R}}
% more definitions for the Annals.
Just change one line in paper.tex to submit to the Annals instead of the Notices.
You're quite right; the book is using the term "domain" incorrectly. What they mean is "the greatest possible subset of the real numbers that could be used as the domain of a function whose values are given by this formula".
As you say, the domain of a function is part of its definition. The "definitions" of $f$ and $g$, in the context of the question, are not definitions at all, but merely equations. I would reformulate the question as: what is the largest domain in $\mathbb{R}$ for which this equation defines a function?