Three new uploads, but ...

Under Lecture notes, I have made three uploads, but it is less to read than it looks. The first one is the big one:

• Tomorrow's topic: calculus of variations. 
• One page with one leftover detail. I will cover it briefly tomorrow.
• A note on how complex numbers work. This is totally optional. Browse it if you think it helps to understand what complex numbers are; otherwise, just do not spend time on it.