If an expression is a function, that means all its x-values in the x,y pairs, are never repeated, the y-values can repeat, but not the x-values
now, an inverse expression of a function, in order to be a function, has to also have unique x-values, x-values that never repeat
however, one characteristic of inverse functions is that, their value pairs, are exactly the same as the original function, but flipped-sideways
namely, if a function has a value pair of 3,5 the inverse will have 5,3
if a function has 7, 11 and 13, 27
the inverse has 11, 7 and 27, 13
so... of all those sets above, if any of those is the value pairs of a function, the x-values must be unique, however, if its inverse expression is also a function, then the y-values must also be unique, namely, if you used the flipped-sideways version, x-values must also be unique
so, will be the one whose x-values AND y-values are unique, let's check