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Mirrors > Home > ILE Home > Th. List > isset | GIF version |
Description: Two ways to say
"𝐴 is a set": A class 𝐴 is a
member of the
universal class V (see df-v 2754)
if and only if the class 𝐴
exists (i.e. there exists some set 𝑥 equal to class 𝐴).
Theorem 6.9 of [Quine] p. 43.
Notational convention: We will use the
notational device "𝐴 ∈ V " to mean "𝐴 is a
set" very
frequently, for example in uniex 4455. Note the when 𝐴 is not
a set,
it is called a proper class. In some theorems, such as uniexg 4457, in
order to shorten certain proofs we use the more general antecedent
𝐴
∈ 𝑉 instead of
𝐴 ∈
V to mean "𝐴 is a set."
Note that a constant is implicitly considered distinct from all variables. This is why V is not included in the distinct variable list, even though df-clel 2185 requires that the expression substituted for 𝐵 not contain 𝑥. (Also, the Metamath spec does not allow constants in the distinct variable list.) (Contributed by NM, 26-May-1993.) |
Ref | Expression |
---|---|
isset | ⊢ (𝐴 ∈ V ↔ ∃𝑥 𝑥 = 𝐴) |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | df-clel 2185 | . 2 ⊢ (𝐴 ∈ V ↔ ∃𝑥(𝑥 = 𝐴 ∧ 𝑥 ∈ V)) | |
2 | vex 2755 | . . . 4 ⊢ 𝑥 ∈ V | |
3 | 2 | biantru 302 | . . 3 ⊢ (𝑥 = 𝐴 ↔ (𝑥 = 𝐴 ∧ 𝑥 ∈ V)) |
4 | 3 | exbii 1616 | . 2 ⊢ (∃𝑥 𝑥 = 𝐴 ↔ ∃𝑥(𝑥 = 𝐴 ∧ 𝑥 ∈ V)) |
5 | 1, 4 | bitr4i 187 | 1 ⊢ (𝐴 ∈ V ↔ ∃𝑥 𝑥 = 𝐴) |
Colors of variables: wff set class |
Syntax hints: ∧ wa 104 ↔ wb 105 = wceq 1364 ∃wex 1503 ∈ wcel 2160 Vcvv 2752 |
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-5 1458 ax-gen 1460 ax-ie1 1504 ax-ie2 1505 ax-8 1515 ax-4 1521 ax-17 1537 ax-i9 1541 ax-ial 1545 ax-ext 2171 |
This theorem depends on definitions: df-bi 117 df-sb 1774 df-clab 2176 df-cleq 2182 df-clel 2185 df-v 2754 |
This theorem is referenced by: issetf 2759 isseti 2760 issetri 2761 elex 2763 elisset 2766 vtoclg1f 2811 ceqex 2879 eueq 2923 moeq 2927 mosubt 2929 ru 2976 sbc5 3001 snprc 3672 snssb 3740 vprc 4150 opelopabsb 4278 eusvnfb 4472 elrelimasn 5012 euiotaex 5212 fvmptdf 5624 fvmptdv2 5626 fmptco 5703 brabvv 5942 ovmpodf 6028 ovi3 6033 tfrlemibxssdm 6352 tfr1onlembxssdm 6368 tfrcllembxssdm 6381 ecexr 6564 snexxph 6979 fnpr2ob 12816 bj-vprc 15109 bj-vnex 15111 bj-2inf 15151 |
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