vertical line test.
A General Function points from each member of "A" to a member of "B".
To be a function you never have one "A" pointing to more than one "B", so one-to-many is not OK in a function (as you would have something like "f(x) = 7 or 9")
Injective, Surjective and Bijective
What about a circle? Is that not a function? If not, why?
The statement that is a function from A to B means:
That last bullet-point is the vertical line test.
Here are some special properties of some functions.
If then the function is surjective.
If then the function is injective.( that is also known as the horizontal line test.)