Idk what to tell you other than that you should check the definitions. A unary function is a special case of a binary relation and in general an n-ary function is a special case of a n+1-ary relation.<p>Edit - Maybe this clarifies it: When we talk about a function we designate one of its variables as the &#x27;output&#x27; and talk about its arity in terms of the inputs. When we talk about a relation there is no distinguished output: A relation is a fully symmetrical definition. That&#x27;s why the arity of a function is one less than the arity of that same function thought of as a relation between its inputs and output.