It can't be the function of a rubber duck though, there are multiple y-values assigned to one x-value, therefore it's not a function.
Sorry to crush your dreams with math.
Number one, all functions must be in y= or f(x)= form, number two, it doesn't matter if it's a valid equation, not all equations are functions. A function is defined by its ability to input one number and output one number.
https://www.mathsisfun.com/definitions/function.html
Sorry to crush your dreams with math.
For 1 x, 2 ys. It's a valid function.
https://www.mathsisfun.com/definitions/function.html