18178 | For Zermelo the successor of n is {n} (rather than n U {n}) [Zermelo, by Maddy] |
15524 | Zermelo's model of arithmetic is distinctive because it rests on a primitive of set theory [Lewis] |
8762 | Two definitions of 3 in terms of sets disagree over whether 1 is a member of 3 [Shapiro] |