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Relating invertibility to being onto and one-to-one

Video transcript
A couple of videos ago, we learned that a function that is a mapping from the set x to the set y is invertible if and only if-- and I'll write that as a if with two f's --if and only if for every y-- so let me write this down. I'll do this in yellow. For every y that is a member of our co-domain, there exists a unique-- and I'll make that a little bit bold --a unique x that is a member of our domain, such that f of this x is equal to this y. So that's just saying that if I take my domain right here, that's x, and then I take a co-domain here, that is y, we say that the function f is invertible. And we know what invertibility means. It means that there's this other function called the inverse that can essentially, if you take that in composition with f, it's like taking the identity on x. Or if you take f in composition with it, it's like taking the identity on y. And we've done that multiple times so I won't repeat that there. We know what invertibility means, but we learned that it's invertible, if and only if, for every y here, so you take any y here, any y that's a member of your co-domain, there exists a unique x, such that f of x is equal to that y. Let me write it this way. If this is an x, let's say that's an x not, f of x not would be equal to y. So this y would be equal to f of x not. You apply the function here. It's going to map it to this point here. It wouldn't be invertible if you had this. If you had two members of x mapping here. That would break invertibility if you had this situation, because then you wouldn't have the unique condition. You have to have a unique x that maps to this thing. And what I just drew here, this other pink mapping, we don't have a unique x that maps to y, we have two x's that map to y. Now, based on what I just told you on that last video, what does this mean? If we have a unique x that maps to each y? That means that we have to have a one-to-one mapping. That f has to be one-to-one. Let me write that. Another way of saying this, is that f is one-to-one, or injective. If we have two guys mapping to the same y, that would break down this condition. We wouldn't be one-to-one and we couldn't say that there exists a unique x solution to this equation right here. Now, the other part of this is that for every y -- you could pick any y here and there exists a unique x that maps to that. There cannot be some y here. Let's say that there's some y here, and no one maps to that. If that's the case, then we don't have our conditions for invertibility. So that would be not invertible. Everything in y, every element of y, has to be mapped to. All of these guys have to be mapped to. And they can only be mapped to by one of the elements of x. Everything here has to be mapped to by a unique guy. Now, in the last video, what did we call it went a function maps to every element of your co-domain? So this every here, what is another way of saying that? That a function maps to every element of your co-domain? On the last video I explained that notion is called a surjective or onto function. So the whole reason why I'm doing this video is because I really just want to restate this condition for invertibility using the vocabulary that I introduced to you in the last video. So given that the statement for every y that's a member of our co-domain, there exists a x that maps to it. We could just say that f is surjective. If we just say that f is surjective, that means that everything here is mapped to. But it could be mapped to, maybe this person right here, could be mapped to by more than one. Surjective doesn't, by itself, make the condition that there's a unique mapping from a member of x to that element of y. So in order to get that, in order to satisfy the unique condition of this condition for invertibility, we have to say that f is also injective. And obviously, maybe the less formal terms for either of these, you call this onto, and you could call this one-to-one. So using the terminology that we learned in the last video, we can restate this condition for invertibility. We can say that a function that is a mapping from the domain x to the co-domain y is invertible, if and only if -- I'll write it out -- f is both surjective and injective. Or we could have said, that f is invertible, if and only if, f is onto and one-to-one. And these are really just fancy ways of saying for every y in our co-domain, there's a unique x that f maps to it. There isn't more than one and every y does get mapped to.