# Function EXACT-DOMAIN

## Slots on this function:

Documentation:
The EXACT-DOMAIN of a relation is a relation whose tuples (all of them) are mapped by the relation to instances of the range. A binary relation R is defined as a set of tuples of form <x,y>. If we say (= (exact-domain R) D) then all of the x's must be in the class D, and for each instance x of class D, the relation maps x to some y. The exact-domain of a relation of arity other than 2 is the relation that represents a cross product. For example, the notation F:A x B -> C, means that function F maps pairs <a,b> onto c's where a is an instance of A, b is an instance of B, and c is an instance of C. The exact-domain of F is the set of pairs <a,b> that occur in some triple <a,b,c> in F.

Some treatments of functions define a function as a mapping from a domain to a range. This ontology treats functions as relations, and relations as sets of tuples. Thus, functions and relations are not defined relative to domains and ranges; the exact-domain is a function of the set of tuples. It follows that all functions are `total' with respect to their exact-domains and `onto' with respect to their exact-ranges.

The exact-domain of a variable-arity relation is another variable-arity relation; the lengths of the tuples in the exact-domain of R is one less than the corresponding tuples in the relation R. The exact-domain of a unary relation, or a relation that contains a tuple of length one, is undefined.

Arity: 2
Domain: Relation
Range: Relation

## Axioms:

```(<=> (Exact-Domain ?Relation ?Domain-Relation)
(And (Relation ?Relation)
(Relation ?Domain-Relation)
(Forall (?Tuple)
(=> (Member ?Tuple ?Relation)
(Not (Empty (Butlast ?Tuple)))))
(Forall (?Tuple @Args)
(<=> (Holds ?Domain-Relation @Args)
(And (Member ?Tuple ?Relation)
(= (Listof @Args) (Butlast ?Tuple)))))))

```

## Notes:

• See-Also: domain exact-range
• Doesn't it have to be a class?

Only for binary relations.

• Why require that the complete domain be included; why not allow for a superset of the true domain?

We need to know the exact-domain of a relation for describing properties such as reflexivity. Supersets of an exact domain are specified with DOMAIN.