We can find a bijection from (0,1) to R. For example, we can use f(x)=2x−11+|2x−1| composed of parts of two hyperbolas, see the graph here. Or we could appropriately scale the tangent function to get g(x)=tanπ(x−12), see the graph here. Several such bijections are suggested in the answers to this post: Is there a bijective map from (0,1) to R?
But does there exist a bijection from [0,1] to R?
If yes, then what is it?
Answer
Let’s fix f:(0,1)→R.
Define g:[0,1]→R as follows:
- g(0)=−1
- g(1)=1
and for $0
- if f(x)∈N∗, then g(x)=f(x)+1
- if −f(x)∈N∗, then g(x)=f(x)−1
- otherwise, g(x)=f(x)
Then, if f is a bijection, so is g.
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