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)=3x−2.
Then I show that f is a bijection by showing that it is injective and surjective.
Answer
Choose an infinite sequence (xn)n⩾1 of distinct elements of (0,1). Let X={xn∣n⩾1}, hence X⊂(0,1). Let x0=1. Define f(xn)=xn+1 for every n⩾0 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 n↦n+1 is a bijection between N∪{0} and N.
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