Mathematics!
First of all, it's maths not math. Sorry Americans.
Here are my posts and notes on mathmatics:
Maths posting and Equations
I talk about posting mathematics and how I setup a system for writing equations easily
The Basel Problem
Discussing the Basel problem and how Euler showed what value it is
Farey Sequences
Adapting a chapter from a number theory book on Farey Sequences
Ye Olde Fractions
Talking about how fractions used to be done
Stellation
Talking about stars geometrically
Trigonometric Functions
Actually considering how trigonometric functions work