Theorem nfsbv

Description: If z is not free in ph , it is not free in [ y / x ] ph when z is distinct from x and y . Version of nfsb requiring more disjoint variables, but fewer axioms. (Contributed by Mario Carneiro, 11-Aug-2016) (Revised by Wolf Lammen, 7-Feb-2023) Remove disjoint variable condition on x , y . (Revised by Steven Nguyen, 13-Aug-2023) (Proof shortened by Wolf Lammen, 25-Oct-2024)

	Ref	Expression
Hypothesis	nfsbv.nf	|- F/ z ph
Assertion	nfsbv	|- F/ z [ y / x ] ph

Proof

Step	Hyp	Ref	Expression
1		nfsbv.nf	|- F/ z ph
2	1	nf5ri	|- ( ph -> A. z ph )
3	2	hbsbw	|- ( [ y / x ] ph -> A. z [ y / x ] ph )
4	3	nf5i	|- F/ z [ y / x ] ph