19/10/27 20:15:07.03 EUeYkluT.net
これ、なかなか面白いわ（＾＾
URLﾘﾝｸ(mathoverflow.net)
MathOverflow
Where did Zermelo first model the natural numbers by iterates of the singleton operator, and have the definitions been compared by himself? asked Jun 29 '17 at 15:32 Peter Heinig