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I know that a continuous function maps compact sets into compact sets. My question now is, are there continuous functions $f:{\mathbb R}\rightarrow I$, with $I=[a,b]$ ($a\neq b$)?

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5  
Constant functions. –  Quinn Culver Jul 31 at 22:19
    
@QuinnCulver: Always the first functions anyone should check when testing out statements or looking for examples/counterexamples. We're on the same page with this. +1. –  MPW Jul 31 at 22:36
    
My guess, @Compa, is that you also want your function to be surjective. If so, you should add that criterion. –  Quinn Culver Jul 31 at 23:18
    
@QuinnCulver Thank you for your comment. Yeah, in fact I was trying to prove something else. I ended up asking this silly question, but it helped me anyway and I appreciate the willingness to help me. –  Compa Aug 1 at 0:48

2 Answers 2

up vote 6 down vote accepted

Take $f(x) = \sin x$. Then $f: \mathbb R \to [-1,1]$.

There are many other similar functions - for example $\frac 2 \pi \tan^{-1} x$ maps $\mathbb R \to (-1,1)$. The point is that whilst it is true that continuous functions take compact sets to compact sets, it is not generally true that the pre-image of a compact set is compact

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Thanks. Wait for 10 minutes for your check mark. –  Compa Jul 31 at 22:24
    
Wait--what? This is incorrect because $\tan x$ is unbounded. Hmm, maybe you mean $\tan^{-1} x$? –  MPW Jul 31 at 22:31
    
Of course. Typo corrected –  Mathmo123 Jul 31 at 22:33
    
Ok, yes, especially with the factor of $2/\pi$ out front, you really couldn't have meant anything else. I'll give you +1. –  MPW Jul 31 at 22:34

Any bounded continuous real-valued function can be scaled and shifted so that its image lies in any given interval of nonzero length.

For example, $f(x) = \tanh x$ maps $\mathbb R$ onto $(-1,1)$. Then $a+ \frac{b-a}{2}\cdot(1+f)$ maps $\mathbb R$ onto $(a,b)\subset [a,b]$

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