>>508-509
＞ 2.(Kazimierz Kuratowski) S has all properties which can be proved by mathematical induction beginning with the empty set and adding one new element at a time. (See below for the set-theoretical formulation of Kuratowski finiteness.)
＞Kuratowski finite means S lies in the set K(S), constructed as follows. Write M for the set of all subsets X of P(S) such that:
＞X contains the empty set;
＞For every set T in P(S), if X contains T then X also contains the union of T with any singleton.
＞Then K(S) may be defined as the intersection of M.

なるほど
”Kuratowski finiteness”の定義では、
CやRやQやNのシングルトン
{C}や{R}や{Q}や{N} 達は
有限集合にはならんな！
思った通りだったな！ｗｗ（＾＾；