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Mirrors > Home > MPE Home > Th. List > Mathboxes > bj-denotes | Structured version Visualization version GIF version |
Description: This would be the
justification theorem for the definition of the unary
predicate "E!" by ⊢ ( E! 𝐴 ↔ ∃𝑥𝑥 = 𝐴) which could be
interpreted as "𝐴 exists" (as a set) or
"𝐴 denotes" (in the
sense of free logic).
A shorter proof using bitri 274 (to add an intermediate proposition ∃𝑧𝑧 = 𝐴 with a fresh 𝑧), cbvexvw 2039, and eqeq1 2735, requires the core axioms and { ax-9 2115, ax-ext 2702, df-cleq 2723 } whereas this proof requires the core axioms and { ax-8 2107, df-clab 2709, df-clel 2809 }. Theorem bj-issetwt 36058 proves that "existing" is equivalent to being a member of a class abstraction. It also requires, with the present proof, { ax-8 2107, df-clab 2709, df-clel 2809 } (whereas with the shorter proof from cbvexvw 2039 and eqeq1 2735 it would require { ax-8 2107, ax-9 2115, ax-ext 2702, df-clab 2709, df-cleq 2723, df-clel 2809 }). That every class is equal to a class abstraction is proved by abid1 2869, which requires { ax-8 2107, ax-9 2115, ax-ext 2702, df-clab 2709, df-cleq 2723, df-clel 2809 }. Note that there is no disjoint variable condition on 𝑥, 𝑦 but the theorem does not depend on ax-13 2370. Actually, the proof depends only on the logical axioms ax-1 6 through ax-7 2010 and sp 2175. The symbol "E!" was chosen to be reminiscent of the analogous predicate in (inclusive or non-inclusive) free logic, which deals with the possibility of nonexistent objects. This analogy should not be taken too far, since here there are no equality axioms for classes: these are derived from ax-ext 2702 and df-cleq 2723 (e.g., eqid 2731 and eqeq1 2735). In particular, one cannot even prove ⊢ ∃𝑥𝑥 = 𝐴 ⇒ ⊢ 𝐴 = 𝐴 without ax-ext 2702 and df-cleq 2723. (Contributed by BJ, 29-Apr-2019.) (Proof modification is discouraged.) |
Ref | Expression |
---|---|
bj-denotes | ⊢ (∃𝑥 𝑥 = 𝐴 ↔ ∃𝑦 𝑦 = 𝐴) |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | bj-denoteslem 36054 | . 2 ⊢ (∃𝑥 𝑥 = 𝐴 ↔ 𝐴 ∈ {𝑧 ∣ ⊤}) | |
2 | bj-denoteslem 36054 | . 2 ⊢ (∃𝑦 𝑦 = 𝐴 ↔ 𝐴 ∈ {𝑧 ∣ ⊤}) | |
3 | 1, 2 | bitr4i 277 | 1 ⊢ (∃𝑥 𝑥 = 𝐴 ↔ ∃𝑦 𝑦 = 𝐴) |
Colors of variables: wff setvar class |
Syntax hints: ↔ wb 205 = wceq 1540 ⊤wtru 1541 ∃wex 1780 ∈ wcel 2105 {cab 2708 |
This theorem was proved from axioms: ax-mp 5 ax-1 6 ax-2 7 ax-3 8 ax-gen 1796 ax-4 1810 ax-5 1912 ax-6 1970 ax-7 2010 ax-8 2107 |
This theorem depends on definitions: df-bi 206 df-an 396 df-tru 1543 df-ex 1781 df-sb 2067 df-clab 2709 df-clel 2809 |
This theorem is referenced by: bj-issettru 36056 bj-issetwt 36058 bj-elissetALT 36060 bj-vtoclg1f1 36101 |
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