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Description: The predicate "is an initial object" of a category, using universal property. (Contributed by Zhi Wang, 23-Oct-2025)
| Ref | Expression | ||
|---|---|---|---|
| Hypotheses | isinito2.1 | |- .1. = ( SetCat ` 1o ) |
|
| isinito2.f | |- F = ( ( 1st ` ( .1. DiagFunc C ) ) ` (/) ) |
||
| Assertion | isinito2 | |- ( I e. ( InitO ` C ) <-> I ( F ( C UP .1. ) (/) ) (/) ) |
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | isinito2.1 | |- .1. = ( SetCat ` 1o ) |
|
| 2 | isinito2.f | |- F = ( ( 1st ` ( .1. DiagFunc C ) ) ` (/) ) |
|
| 3 | initorcl | |- ( I e. ( InitO ` C ) -> C e. Cat ) |
|
| 4 | eqid | |- ( Base ` C ) = ( Base ` C ) |
|
| 5 | 4 | initoo2 | |- ( I e. ( InitO ` C ) -> I e. ( Base ` C ) ) |
| 6 | 1 2 3 5 | isinito2lem | |- ( I e. ( InitO ` C ) -> ( I e. ( InitO ` C ) <-> I ( F ( C UP .1. ) (/) ) (/) ) ) |
| 7 | 6 | ibi | |- ( I e. ( InitO ` C ) -> I ( F ( C UP .1. ) (/) ) (/) ) |
| 8 | id | |- ( I ( F ( C UP .1. ) (/) ) (/) -> I ( F ( C UP .1. ) (/) ) (/) ) |
|
| 9 | 8 | up1st2nd | |- ( I ( F ( C UP .1. ) (/) ) (/) -> I ( <. ( 1st ` F ) , ( 2nd ` F ) >. ( C UP .1. ) (/) ) (/) ) |
| 10 | 9 | uprcl2 | |- ( I ( F ( C UP .1. ) (/) ) (/) -> ( 1st ` F ) ( C Func .1. ) ( 2nd ` F ) ) |
| 11 | 10 | funcrcl2 | |- ( I ( F ( C UP .1. ) (/) ) (/) -> C e. Cat ) |
| 12 | 9 4 | uprcl4 | |- ( I ( F ( C UP .1. ) (/) ) (/) -> I e. ( Base ` C ) ) |
| 13 | 1 2 11 12 | isinito2lem | |- ( I ( F ( C UP .1. ) (/) ) (/) -> ( I e. ( InitO ` C ) <-> I ( F ( C UP .1. ) (/) ) (/) ) ) |
| 14 | 13 | ibir | |- ( I ( F ( C UP .1. ) (/) ) (/) -> I e. ( InitO ` C ) ) |
| 15 | 7 14 | impbii | |- ( I e. ( InitO ` C ) <-> I ( F ( C UP .1. ) (/) ) (/) ) |