Hi there, welcome to “Back To The Basics”. I am toto, this is my personal blog which is used to note fundamental topics on mathematics and computer science, maybe also some complicated theories for myself. Note anything that is interesting. Some of the articles are written in English, some are in Chinese denpending on the specific topics. If you find something you are interested in, please feel free to contact me by email toto.zhang@gmail.com (mails not checked regularly) or toto.zhang@me.com. Have a nice day. And I’am really glad that some of these notes are helpful to some friends, please feel free to use it if it’s helpful.