a busy fool I sometimes encounter a brieft but uncomplete author's notes in the book. I think those are from some related fields but no exactly the topic of the book when I find those are frequently mentioned in other books in other fields. But I don't like an uncomplete notion without backgrounds of sufficient sufficient proof or explanation. I forget formulas easily if there are no proofs. I don't feel good. I get asked some questions how to solve problems using the formulas. So I wanted to find some good proofs or at least good explanations, but it's no use searching in other books. It was grueling, but I got no products.