# Metamath Proof Explorer

## Definition df-fun

Description: Define predicate that determines if some class A is a function. Definition 10.1 of Quine p. 65. For example, the expression Fun cos is true once we define cosine ( df-cos ). This is not the same as defining a specific function's mapping, which is typically done using the format of cmpt with the maps-to notation (see df-mpt and df-mpo ). Contrast this predicate with the predicates to determine if some class is a function with a given domain ( df-fn ), a function with a given domain and codomain ( df-f ), a one-to-one function ( df-f1 ), an onto function ( df-fo ), or a one-to-one onto function ( df-f1o ). For alternate definitions, see dffun2 , dffun3 , dffun4 , dffun5 , dffun6 , dffun7 , dffun8 , and dffun9 . (Contributed by NM, 1-Aug-1994)

Ref Expression
Assertion df-fun
`|- ( Fun A <-> ( Rel A /\ ( A o. `' A ) C_ _I ) )`

### Detailed syntax breakdown

Step Hyp Ref Expression
0 cA
` |-  A`
1 0 wfun
` |-  Fun A`
2 0 wrel
` |-  Rel A`
3 0 ccnv
` |-  `' A`
4 0 3 ccom
` |-  ( A o. `' A )`
5 cid
` |-  _I`
6 4 5 wss
` |-  ( A o. `' A ) C_ _I`
7 2 6 wa
` |-  ( Rel A /\ ( A o. `' A ) C_ _I )`
8 1 7 wb
` |-  ( Fun A <-> ( Rel A /\ ( A o. `' A ) C_ _I ) )`