Monday, 25 January 2016

real analysis - How to define a bijection between (0,1) and (0,1]?





How to define a bijection between (0,1) and (0,1]?
Or any other open and closed intervals?




If the intervals are both open like (1,2) and (5,4) I do a cheap trick (don't know if that's how you're supposed to do it):
I make a function f:(1,2)(5,4) of the form f(x)=mx+b by
5=f(1)=m(1)+b4=f(2)=m(2)+b


Solving for m and b I find m=3 and b=2 so then f(x)=3x2.



Then I show that f is a bijection by showing that it is injective and surjective.


Answer



Choose an infinite sequence (xn)n1 of distinct elements of (0,1). Let X={xnn1}, hence X(0,1). Let x0=1. Define f(xn)=xn+1 for every n0 and f(x)=x for every x in (0,1)X. Then f is defined on (0,1] and the map f:(0,1](0,1) is bijective.



To sum up, one extracts a copy of N from (0,1) and one uses the fact that the map nn+1 is a bijection between N{0} and N.


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