Not a function: “one to many“. In other words, let’s say you had one x-value that maps to many y-values. For example, — in coordinate notation — (2,1) and (2,10). If the first number (the x-value) repeats, then you do not have a function. To put that another way, ...
This function $$\frac{e^x - e^{-x}}{2}$$ is called the hyperbolic sine. I'll refer to it as "hypersine" for short. You could say it "splits the...