I don't really understand multi-valued function. I hope one of you can make me understand it. What I've learned from google, I suppose that a multi-valued function is a binary relation that maps the domain more than once to a single point (i can't describe it well).
So, here, can I conveniently claim that a circle (on the real plane) is a multi-valued function since from one point on the domain we can get $2$ points as the range?
Thanks in advance.
Best Answer
The circle $S$ can be viewed as a binary relation on $\Bbb{R}$, simply because it is a subset of $\Bbb{R}^2$. And indeed if $(x,y)\in S$ then also $(x,-y)\in S$, so if $y\neq0$ then this shows that $S$ is multivalued. But $S$ is not a function on $\Bbb{R}$, because it is not defined on all of $\Bbb{R}$; there is no $y\in\Bbb{R}$ such that $(2,y)\in S$, for example. So the circle cannot be viewed as a multivalued function on $\Bbb{R}$.
There are two ways to adjust your example to valid example of a multivalued function: