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Theorem equid1 2099
Description: Identity law for equality (reflexivity). Lemma 6 of [Tarski] p. 68. This is often an axiom of equality in textbook systems, but we don't need it as an axiom since it can be proved from our other axioms (although the proof, as you can see below, is not as obvious as you might think). This proof uses only axioms without distinct variable conditions and thus requires no dummy variables. A simpler proof, similar to Tarki's, is possible if we make use of ax-17 1604; see the proof of equid 1646. See equid1ALT 2117 for an alternate proof. (Contributed by NM, 5-Aug-1993.) (Proof modification is discouraged.) (New usage is discouraged.)
Assertion
Ref Expression
equid1  |-  x  =  x

Proof of Theorem equid1
StepHypRef Expression
1 ax-5o 2078 . . . 4  |-  ( A. x ( A. x  -.  A. x  x  =  x  ->  ( x  =  x  ->  A. x  x  =  x )
)  ->  ( A. x  -.  A. x  x  =  x  ->  A. x
( x  =  x  ->  A. x  x  =  x ) ) )
2 ax-4 2077 . . . . 5  |-  ( A. x  -.  A. x  x  =  x  ->  -.  A. x  x  =  x )
3 ax-12o 2084 . . . . 5  |-  ( -. 
A. x  x  =  x  ->  ( -.  A. x  x  =  x  ->  ( x  =  x  ->  A. x  x  =  x )
) )
42, 2, 3sylc 58 . . . 4  |-  ( A. x  -.  A. x  x  =  x  ->  (
x  =  x  ->  A. x  x  =  x ) )
51, 4mpg 1536 . . 3  |-  ( A. x  -.  A. x  x  =  x  ->  A. x
( x  =  x  ->  A. x  x  =  x ) )
6 ax-9o 2080 . . 3  |-  ( A. x ( x  =  x  ->  A. x  x  =  x )  ->  x  =  x )
75, 6syl 17 . 2  |-  ( A. x  -.  A. x  x  =  x  ->  x  =  x )
8 ax-6o 2079 . 2  |-  ( -. 
A. x  -.  A. x  x  =  x  ->  x  =  x )
97, 8pm2.61i 158 1  |-  x  =  x
Colors of variables: wff set class
Syntax hints:   -. wn 5    -> wi 6   A.wal 1528
This theorem was proved from axioms:  ax-1 7  ax-2 8  ax-3 9  ax-mp 10  ax-gen 1534  ax-4 2077  ax-5o 2078  ax-6o 2079  ax-9o 2080  ax-12o 2084
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