# Continuous bijection from $(0,1)$ to $[0,1]$

Does there exist a continuous bijection from $(0,1)$ to $[0,1]$? Of course the map should not be a proper map.

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@Asaf: I don't think that this really is a duplicate, even if the question is briefly addressed there. –  t.b. May 31 '11 at 11:33
@Theo: You are correct. :-) –  Asaf Karagila May 31 '11 at 11:41
What about such map from a non compact set to compact set in nice topologies? –  Alex May 31 '11 at 12:14
what exactly is the reason for non existence? In the (0,1) case some sort of local compactness is the reason.Can the proof generalized to non existence of a map from open ball in R^n to a closed ball. –  Alex May 31 '11 at 12:27
@Alex: By the way, since you're new here: you can accept an answer by clicking on the grey checkmark sign to the right of it (I'm not saying you should accept mine even though I wouldn't complain). –  t.b. May 31 '11 at 12:53

No. If $f:(0,1) \to [0,1]$ were continuous and bijective, there would be a unique point $x \in (0,1)$ such that $f(x) = 1$. However, since $f$ is continuous, the intervals $[x - \varepsilon, x]$ and $[x, x + \varepsilon]$ would be mapped to intervals $[a,1]$ and $[b,1]$, say. By bijectivity we'd have $a, b \lt 1$. Thus every value strictly between $\max{\{a,b\}}$ and $1$ would be assumed at least twice, contradicting bijectivity.

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Let $f:(0,1) \rightarrow [0,1]$ be continuous and surjective. (Actually, we just need to suppose that $0$ and $1$ are in the image of $f$.) Let $a,b \in (0,1)$ such that $f(a)=0$ and $f(b)=1$. Let $I=[a,b]$ if $a<b$ or $I=[b,a]$ if $b<a$. Then, by the intermediate value theorem, $f(I)$ is an interval that contains $0$ and $1$ and so $f(I)$ contains $[0,1]$, which implies $f(I)=[0,1]$. But then $f$ cannot be injective because there are lots of points in $(0,1)\setminus I$.

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I like this answer a lot :) Very clever! –  Prism May 5 at 22:10
+1 thanks dear friend –  B.S. Jun 30 at 11:24

Suppose that $f:(0,1) \rightarrow [0,1]$ is 1-1 and continuous. By the intermediate value theorem, the image of any interval under $f$ is an interval. Since $f$ is 1-1, it is either (strictly) monotone increasing or decreasing. Hence, $f(0,1)$ is an interval. Without loss of generality, assume $f$ is increasing; were it not this analysis would apply to $1 - f$.

Suppose now that $f$ is onto; then we must have some $t\in(0,1)$ with $f(t) = 1$. Because $f$ is strictly monotone increasing, we would have to have $f(s) > 1$, for $t \le s < 1$. This violates the premise that $f(0,1) \subseteq [0,1]$. Hence, $f$ cannot be onto.

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You need the continuity to get this. 1-1 alone does not imply monotone. –  ncmathsadist May 31 '11 at 12:20
I don't think a continuous 1-1 function should be monotone.There may be non differentiable kind of things with nowhere monotonocity –  Alex May 31 '11 at 12:20
Yes, continuous 1-1 function defined on an interval is monotone. Interesting application of intermediate value theorem several times. –  GEdgar May 31 '11 at 13:22

Since Theo gave an answer I am going to be nitpicking and add one remark. When speaking about continuity (especially when tagging under [topology]) it is best to mention the topology you are working with. In this case, you mean in the standard topology.

Otherwise, consider the discrete topology, i.e. every set is open:

Let $f\colon [0,1]\to (0,1)$ be any bijection, it is continuous since all sets are open, the preimage of an open set is an open set, thus $f$ is continuous.

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Oy, do you really want me not to downvote this? :) Of course, you're right, but come on! :), +1 for the effort –  t.b. May 31 '11 at 11:49
On the contrary: when mentioning subsets of the reals, assume the standard topology unless otherwise stated. –  GEdgar May 31 '11 at 13:24
@GEdgar: When taking a course in real analysis? Sure. When taking a course in point-set topology? Not if you want to be accurate. –  Asaf Karagila May 31 '11 at 14:00
@Asaf: When answering a question on this board? YES. Or, if you want to be super-accurate, add "assuming the usual topology" to your answer. –  GEdgar Jun 1 '11 at 13:25
@AsafKaragila: it would be complete to give an example of one such bijection (e.g. $0\mapsto\frac12$, $\frac1n\mapsto\frac1{n+2}$ for $n=1,2,3,\dots$, and $x\mapsto x$, otherwise). –  robjohn Nov 4 '12 at 15:28