I have the answer to Determine wheather the given g is a function with domain X and some codomain Y. However, I do not understand it. I believe if g does not exist for a X then it is not in the domain

1) X is the set of real numbers and, for X, g(x) = log₂x.

Not in the domain

2) X is the set of real numbers and, for X, g(x) = x2^x

Not in the domain

3) X is the set of students at Illinois State University and, for X, g(x) is the oldest brother x.