Countable Sets and Hessenberg's Theorem
Grzegorz Bancerek · 1991
Summary. The concept of countable sets is introduced and there are shown some facts which deal with finite and countable sets. Besides, the article includes theorems and lemmas on the sum and the product of infinite cardinals. The most important of them is Hessenberg’s theorem which says that for every infinite cardinal m the product m · m is equal to m.