Here are notes on some things I have thought about, sorted by subject. I feel that it is very easy to forget small things we learn in maths. Sometimes it can be a nuisance to track down specific statements in books, so it is useful to collect them in one place.
These are for personal uses only. I do not claim originality for the content of these notes. If you find any mathematical errors, please email me at ziheng.huang@warwick.ac.uk