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Mirrors > Home > ILE Home > Th. List > ax-i12 | GIF version |
Description: Axiom of Quantifier
Introduction. One of the equality and substitution
axioms of predicate calculus with equality. Informally, it says that
whenever 𝑧 is distinct from 𝑥 and
𝑦,
and 𝑥 =
𝑦 is true,
then 𝑥 = 𝑦 quantified with 𝑧 is also
true. In other words, 𝑧
is irrelevant to the truth of 𝑥 = 𝑦. Axiom scheme C9' in [Megill]
p. 448 (p. 16 of the preprint). It apparently does not otherwise appear
in the literature but is easily proved from textbook predicate calculus by
cases.
This axiom has been modified from the original ax-12 1474 for compatibility with intuitionistic logic. (Contributed by Mario Carneiro, 31-Jan-2015.) |
Ref | Expression |
---|---|
ax-i12 | ⊢ (∀𝑧 𝑧 = 𝑥 ∨ (∀𝑧 𝑧 = 𝑦 ∨ ∀𝑧(𝑥 = 𝑦 → ∀𝑧 𝑥 = 𝑦))) |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | vz | . . . 4 setvar 𝑧 | |
2 | vx | . . . 4 setvar 𝑥 | |
3 | 1, 2 | weq 1464 | . . 3 wff 𝑧 = 𝑥 |
4 | 3, 1 | wal 1314 | . 2 wff ∀𝑧 𝑧 = 𝑥 |
5 | vy | . . . . 5 setvar 𝑦 | |
6 | 1, 5 | weq 1464 | . . . 4 wff 𝑧 = 𝑦 |
7 | 6, 1 | wal 1314 | . . 3 wff ∀𝑧 𝑧 = 𝑦 |
8 | 2, 5 | weq 1464 | . . . . 5 wff 𝑥 = 𝑦 |
9 | 8, 1 | wal 1314 | . . . . 5 wff ∀𝑧 𝑥 = 𝑦 |
10 | 8, 9 | wi 4 | . . . 4 wff (𝑥 = 𝑦 → ∀𝑧 𝑥 = 𝑦) |
11 | 10, 1 | wal 1314 | . . 3 wff ∀𝑧(𝑥 = 𝑦 → ∀𝑧 𝑥 = 𝑦) |
12 | 7, 11 | wo 682 | . 2 wff (∀𝑧 𝑧 = 𝑦 ∨ ∀𝑧(𝑥 = 𝑦 → ∀𝑧 𝑥 = 𝑦)) |
13 | 4, 12 | wo 682 | 1 wff (∀𝑧 𝑧 = 𝑥 ∨ (∀𝑧 𝑧 = 𝑦 ∨ ∀𝑧(𝑥 = 𝑦 → ∀𝑧 𝑥 = 𝑦))) |
Colors of variables: wff set class |
This axiom is referenced by: ax-12 1474 ax12or 1475 dveeq2 1771 dveeq2or 1772 dvelimALT 1963 dvelimfv 1964 |
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