Wednesday, 12 March 2014

functions - Explicit bijection between [0,1) and (0,1)





Proving that [0,1) and (0,1) have the same cardinality (without assuming any previous knowledge) can be done easily using Cantor-Bernstein theorem.



However I'm wondering if someone can build an explicit bijection between these sets.



It's easy to build a bijection between (0,1) and R, so a bijection from [0,1) to R will also fit the bill.


Answer



Let us partition (0,1) into a countable number of disjoint subsets of the form [1n+1,1n) for n=0,1,2,.



These half-open intervals may then be positioned in reverse order to form a half-open interval of equal length. Whether this construction is sufficiently explicit is open to question, but it does allow the relocation of any x(0,1) to [0,1) to be computed in short order.




A more succinct construction is to define f:[0,1)(0,1) by f(0)=1/2, f(1/n)=1/(n+1) for integer n2, and f(x)=x otherwise.


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