books.google.com - Every mathematician agrees that every mathematician must know some set theory; the disagreement begins in trying to decide how much is some. This book contains my answer to that question. The purpose of the book is to tell the beginning student of advanced mathematics the basic set theoretic facts of...http://books.google.com/books/about/Naive_Set_Theory.html?id=x6cZBQ9qtgoC&q=ordinal+sum&utm_source=gb-gplus-shareNaive Set Theory