I'm asked to find an explicit formula for a bijection f: (-1,1) -> R
And I know this formula f(x) = x/(x^2-1) works and I've verified it using a graphing program.
But how do I prove that this function is 1-1 and onto using the definition? It seems like the x and the x^2 throws me off. Thanks
Suppose that - 1,1) \to \mathbb{R};~~x \mapsto \frac{x}{{x^2 - 1}}" alt=" f - 1,1) \to \mathbb{R};~~x \mapsto \frac{x}{{x^2 - 1}}" />.
To show that the function is injective suppose that then show that .
Note that if then it must be the case that have the same sign.
And from the equality we get .
What is wrong with that? (Remember they have same sign)
Does that mean that
To do the surjective part, for all does have a solution in
I don't follow you on the injective part. Please explain.
Also, I've proven linear functions using the definition of 1-1 and onto with ease, but it seems like this function is a lot more complicated to prove, especially the onto part where you have to show that the y is in the domain (-1,1).
The domain of your function consists of a point in a plane. While the function being 1-1, the point in the domain has exactly one image in the real number line. In other words, the range of the function is not equal to the codomain, which should tell you something.
Here, I want to define my own function say . The domain consists of only one point. I would check to see whether the image is unique. See if it's . This implies that , which means that and . This also implies that the function is 1-1. This should not be hard since it involves only plugging something into a machine, just like putting money into a vending machine and get what you wanted. If you put money into a vending machine wishing to get an item and end up getting two, then you know the machine is out of order.
For unto function, the range of the function must equal to the codomain. Now, suppose that I am the owner of the vending machine. I have all my items displayed, say 50 cans of soda, and expect a dollar for each can, but I have only customer who has only a dollar. All I could do is sell a can of soda. I have no takers for the remaining 49 cans of soda. That's bad new, which I would say, my business is not onto my expectation.
Of course, no one would pay a dollar for a can of soda. Extortion, eh?
I realized the lines in blue was a poor example for a 1-1 function. In fact, it's an example of something not being a function.
The correction example of a 1-1 function is that of a machine containing distinct items for each asking price. For the first coin you drop, you get one item, and when you drop the second coin the item must not be similar to the first one you get.
Professor Plato is a very good teacher. I hope I am not making a fool out of myself.