Tuesday, October 4, 2016

elementary set theory - Does there exist a bijection from [0,1] to mathbbR?



We can find a bijection from (0,1) to R. For example, we can use f(x)=2x11+|2x1| composed of parts of two hyperbolas, see the graph here. Or we could appropriately scale the tangent function to get g(x)=tanπ(x12), 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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