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Bijection between an open and a closed interval

I'm a little confused, maybe i'm over thinking something so basic. But a closed interval [a,b] of real numbers has an infinite number of elements, correct? And the same goes for (a,b)? Then, [a,b] has the same cardinality as (a,b)?

Thanks.

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    Thank you. I was a little confused because my instructor said "finite intervals" when describing them.2012-10-30

2 Answers 2

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Yes, they have same cardinality. For convenience, here is a bijection $[0,1]\to[0,1)$:

$f(x)=\begin{cases}\frac1{n+1}&\text{if }x=\frac1n\text{ for some }n\in\mathbb N\\x&\text{otherwise}\end{cases}$

Then

$g(x)=f(1-f(x))$ is a bijection $[0,1]\to(0,1)$.

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Yes, just as the integers greater than 0 have the same cardinality as the integers greater than 1. Deleting a finite number of elements from any infinite set doesn't change its cardinality. In this case we can find an explicit bijection. Define $f(x): [0,1) \to (0,1)$ as follows: If $x \ne 0, \frac 1{3^k}, f(x)=x \\ f(0)=\frac 13 \\f(\frac 1{3^k})=\frac 1{3^{k+1}}$

You can do the same at the other end.