Determine whether each of the relations $f=\{(a,b), (c,d), (e,f), (g,h), (i,j)\}$ is a function with domain $\{1,2,3,4\}$ 1.) Determine whether each of the following relations is a function with domain $\{1,2,3,4\}$. For any relation that is not a function, explain why it isn't.
a.) $f=\{(1,1), (2,1), (3,1), (4,1), (3,3)\}$. - The answer in back of the book states the following: "Not a function; $f$ contains two different pairs of the form $(3,-)$." What does the dash mean?
b.) $f=\{(1,2), (2,3), (4,2)\}$ - ?
 d.) $f=\{(1,1), (1,2), (1,3), (1,4)\}$ - ?
 e.) $f=\{(1,4), (2,3), (3,2), (4,1)\}$ - ? 

A function is well defined, that means $x=y\implies f(x)=f(y)$  but you have once $f(3)=1$ and $f(3)=3$
try for the rst. for your control, b is no function, d is no function but e is a function.
In b) you have no function because $f(3)$ does not exist, but a function assigns to every object of your domain, an object of your codomain.
