Theorem rankpr

Description: The rank of an unordered pair. Part of Exercise 30 of Enderton p. 207. (Contributed by NM, 28-Nov-2003) (Revised by Mario Carneiro, 17-Nov-2014)

	Ref	Expression
Hypotheses	ranksn.1	⊢ 𝐴 ∈ V
	rankun.2	⊢ 𝐵 ∈ V
Assertion	rankpr	⊢ ( rank ‘ { 𝐴 , 𝐵 } ) = suc ( ( rank ‘ 𝐴 ) ∪ ( rank ‘ 𝐵 ) )

Proof

Step	Hyp	Ref	Expression
1		ranksn.1	⊢ 𝐴 ∈ V
2		rankun.2	⊢ 𝐵 ∈ V
3		unir1	⊢ ∪ ( 𝑅1 “ On ) = V
4	1 3	eleqtrri	⊢ 𝐴 ∈ ∪ ( 𝑅1 “ On )
5	2 3	eleqtrri	⊢ 𝐵 ∈ ∪ ( 𝑅1 “ On )
6		rankprb	⊢ ( ( 𝐴 ∈ ∪ ( 𝑅1 “ On ) ∧ 𝐵 ∈ ∪ ( 𝑅1 “ On ) ) → ( rank ‘ { 𝐴 , 𝐵 } ) = suc ( ( rank ‘ 𝐴 ) ∪ ( rank ‘ 𝐵 ) ) )
7	4 5 6	mp2an	⊢ ( rank ‘ { 𝐴 , 𝐵 } ) = suc ( ( rank ‘ 𝐴 ) ∪ ( rank ‘ 𝐵 ) )