In latex you can add footnotes and margin annotations but...
How can you easily add notes just in the region of interest, below or above the line? Maybe it's called "inline" notes.
For example something like this:

I mean notes that are also going to be printed, as the rest of the document.
Something simple like "underbrace" but with an arrow instead of a brace would be great. How?
a\cdot \underbrace{a}_{b\text{ times}}\cdots a
$a\cdot \underbrace{a}_{b\text{ times}}\cdots a$


arrow equation)? Check for example http://tex.stackexchange.com/questions/145670/highlighting-equation-with-arrow – Mensch Oct 16 '15 at 13:53