Description: Existence of union is equivalent to existence of its components. (Contributed by NM, 11-Jun-1998)
Ref | Expression | ||
---|---|---|---|
Assertion | unexb | |- ( ( A e. _V /\ B e. _V ) <-> ( A u. B ) e. _V ) |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | unexg | |- ( ( A e. _V /\ B e. _V ) -> ( A u. B ) e. _V ) |
|
2 | ssun1 | |- A C_ ( A u. B ) |
|
3 | ssexg | |- ( ( A C_ ( A u. B ) /\ ( A u. B ) e. _V ) -> A e. _V ) |
|
4 | 2 3 | mpan | |- ( ( A u. B ) e. _V -> A e. _V ) |
5 | ssun2 | |- B C_ ( A u. B ) |
|
6 | ssexg | |- ( ( B C_ ( A u. B ) /\ ( A u. B ) e. _V ) -> B e. _V ) |
|
7 | 5 6 | mpan | |- ( ( A u. B ) e. _V -> B e. _V ) |
8 | 4 7 | jca | |- ( ( A u. B ) e. _V -> ( A e. _V /\ B e. _V ) ) |
9 | 1 8 | impbii | |- ( ( A e. _V /\ B e. _V ) <-> ( A u. B ) e. _V ) |