Description: The empty set is an ordinal number. Corollary 7N(b) of Enderton p. 193. Remark 1.5 of Schloeder p. 1. (Contributed by NM, 17-Sep-1993)
| Ref | Expression | ||
|---|---|---|---|
| Assertion | 0elon | |- (/) e. On |
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | ord0 | |- Ord (/) |
|
| 2 | 0ex | |- (/) e. _V |
|
| 3 | 2 | elon | |- ( (/) e. On <-> Ord (/) ) |
| 4 | 1 3 | mpbir | |- (/) e. On |