Theorem hbequid 1527

Description: Bound-variable hypothesis builder for x = x. This theorem tells us that any variable, including x, is effectively not free in x = x, even though x is technically free according to the traditional definition of free variable.

The proof uses only ax-8 1518 and ax-i12 1521 on top of (the FOL analogue of) modal logic KT. This shows that this can be proved without ax-i9 1544, even though Theorem equid 1715 cannot. A shorter proof using ax-i9 1544 is obtainable from equid 1715 and hbth 1477. (Contributed by NM, 13-Jan-2011.) (Proof shortened by Wolf Lammen, 23-Mar-2014.)

Assertion

Ref	Expression
hbequid	|- ( x = x -> A. y x = x )

Proof of Theorem hbequid

Step	Hyp	Ref	Expression
1		ax12or 1522	. 2 |- ( A. y y = x \/ ( A. y y = x \/ A. y ( x = x -> A. y x = x ) ) )
2		ax-8 1518	. . . . . 6 |- ( y = x -> ( y = x -> x = x ) )
3	2	pm2.43i 49	. . . . 5 |- ( y = x -> x = x )
4	3	alimi 1469	. . . 4 |- ( A. y y = x -> A. y x = x )
5	4	a1d 22	. . 3 |- ( A. y y = x -> ( x = x -> A. y x = x ) )
6		ax-4 1524	. . . 4 |- ( A. y ( x = x -> A. y x = x ) -> ( x = x -> A. y x = x ) )
7	5, 6	jaoi 717	. . 3 |- ( ( A. y y = x \/ A. y ( x = x -> A. y x = x ) ) -> ( x = x -> A. y x = x ) )
8	5, 7	jaoi 717	. 2 |- ( ( A. y y = x \/ ( A. y y = x \/ A. y ( x = x -> A. y x = x ) ) ) -> ( x = x -> A. y x = x ) )
9	1, 8	ax-mp 5	1 |- ( x = x -> A. y x = x )

Syntax hints:   -> wi 4   \/ wo 709   A.wal 1362   = wceq 1364

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 106  ax-ia2 107  ax-ia3 108  ax-io 710  ax-5 1461  ax-gen 1463  ax-8 1518  ax-i12 1521  ax-4 1524

This theorem depends on definitions:  df-bi 117

This theorem is referenced by:  equveli  1773