Theorem bdaydm

Description: The birthday function's domain is No . (Contributed by Scott Fenton, 14-Jun-2011)

	Ref	Expression
Assertion	bdaydm	⊢ dom bday = No

Proof

Step	Hyp	Ref	Expression
1		bdayfo	⊢ bday : No –onto→ On
2		fof	⊢ ( bday : No –onto→ On → bday : No ⟶ On )
3	1 2	ax-mp	⊢ bday : No ⟶ On
4	3	fdmi	⊢ dom bday = No