Full Idea
For Von Neumann the successor of n is n U {n} (rather than Zermelo's successor, which is {n}).
Gist of Idea
For Von Neumann the successor of n is n U {n} (rather than {n})
Source
report of John von Neumann (On the Introduction of Transfinite Numbers [1923]) by Penelope Maddy - Naturalism in Mathematics I.2 n8
Book Reference
Maddy,Penelope: 'Naturalism in Mathematics' [OUP 2000], p.24