How do I show that all continuous periodic functions are bounded and uniform continuous?

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A function $f:mathbb{R}to mathbb{R}$ is periodic if there exits $p>0$ such that $f(x+P)=f(x)$ for all $xin mathbb{R}$. Show that every continuous periodic function is bounded and uniformly continuous.
For boundedness, I first tried to show that since the a periodic function is continuous, it is continuous for the closed interval $[x_0,x_0+P]$. I know that there is a theorem saying that if it is continuous on a closed interval, then it is bounded. However, I'm not allowed to state that theorem directly. Should I just aim for a contradiction by supposing f is not bounded on the interval stated above?
real-analysis continuity uniform-continuity periodic-functions
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A function $f:mathbb{R}to mathbb{R}$ is periodic if there exits $p>0$ such that $f(x+P)=f(x)$ for all $xin mathbb{R}$. Show that every continuous periodic function is bounded and uniformly continuous.
For boundedness, I first tried to show that since the a periodic function is continuous, it is continuous for the closed interval $[x_0,x_0+P]$. I know that there is a theorem saying that if it is continuous on a closed interval, then it is bounded. However, I'm not allowed to state that theorem directly. Should I just aim for a contradiction by supposing f is not bounded on the interval stated above?
real-analysis continuity uniform-continuity periodic-functions
$endgroup$
add a comment |
$begingroup$
A function $f:mathbb{R}to mathbb{R}$ is periodic if there exits $p>0$ such that $f(x+P)=f(x)$ for all $xin mathbb{R}$. Show that every continuous periodic function is bounded and uniformly continuous.
For boundedness, I first tried to show that since the a periodic function is continuous, it is continuous for the closed interval $[x_0,x_0+P]$. I know that there is a theorem saying that if it is continuous on a closed interval, then it is bounded. However, I'm not allowed to state that theorem directly. Should I just aim for a contradiction by supposing f is not bounded on the interval stated above?
real-analysis continuity uniform-continuity periodic-functions
$endgroup$
A function $f:mathbb{R}to mathbb{R}$ is periodic if there exits $p>0$ such that $f(x+P)=f(x)$ for all $xin mathbb{R}$. Show that every continuous periodic function is bounded and uniformly continuous.
For boundedness, I first tried to show that since the a periodic function is continuous, it is continuous for the closed interval $[x_0,x_0+P]$. I know that there is a theorem saying that if it is continuous on a closed interval, then it is bounded. However, I'm not allowed to state that theorem directly. Should I just aim for a contradiction by supposing f is not bounded on the interval stated above?
real-analysis continuity uniform-continuity periodic-functions
real-analysis continuity uniform-continuity periodic-functions
edited Jan 14 at 13:07


Martin Sleziak
44.8k9118272
44.8k9118272
asked Apr 30 '14 at 1:13
Oscar FloresOscar Flores
623618
623618
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3 Answers
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Suppose that $f$ has period one. Since $f:[0,2]toBbb R$ is continuous, it is bounded, so $f$ is bounded all over $Bbb R$ (why?). Also, $f:[0,2]toBbb R$ is uniformly continuous, being continuous on a compact set. Thus, given $varepsilon >0$ there exists $delta>0$ such that, whenever $|x-y|<delta,x,yin[0,2]$, then $|f(x)-f(y)|<varepsilon$. Pick arbitrary $x,y$, and assume $x<y$ with $|x-y|<delta$. We may take $delta <1$. I claim there is an integer $n$ such that $x-n,y-nin [0,2]$. Then $|x-y|=|x-n-(y-n)|$ and $f(x-n)=f(x)$, $f(y-n)=f(y)$. Can you continue now?
Drawing a picture would prove useful. Essentially, you're translating the problem to $[0,2]$ where we already solved the issue.
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$begingroup$
ah, I was trying to deal with the case where, say, we have $x= 0.99$ and $y = 1.01$. Considering $f$ on $[0,2]$ instead of $[0,1]$ is a smart way to deal with that!
$endgroup$
– MCT
Mar 4 '15 at 22:06
add a comment |
$begingroup$
A periodic continuous function is simply a function defined on a circle,
$$f:S^1 longrightarrow R.$$
Circle is a compact space. A continuous function on a compact space is both bounded and uniformly continuous.
$endgroup$
$begingroup$
But $f$ is defined on $mathbb R$, not on the circle.
$endgroup$
– Al Jebr
Jun 24 '18 at 18:54
$begingroup$
AI Jebr: We can identify a periodic function with one defined on a circle. You could also argue by noting that the image of a periodic function is the same as its image when restricted to one cycle.
$endgroup$
– Behnam Esmayli
Jun 24 '18 at 22:38
add a comment |
$begingroup$
Here's proof for boundedness part by contradiction :
Fix $a in Bbb R$. Consider the domain $I=[a,a+p]$. Assume $f$ is not bounded. Then there exists a sequence $x_n in I$ such that $|f(x_n)| gt n$.
Since $x_n$ is bounded and consists of real numbers, it has a convergent subsequence $x_{n_k}$. But $I$ is closed hence $x_{n_k}$ converges inside $I$. i.e. $exists x in I$ such that $x_{n_k} to x$.
What we have got is a sequence $x_{n_k} to x in I$ and $|f(x_{n_k})| to infty ; (because |f(x_{n_k})| gt n_k)$. This is contradiction for $f$ being continuous on $I$ and hence at $x$.
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3 Answers
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3 Answers
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$begingroup$
Suppose that $f$ has period one. Since $f:[0,2]toBbb R$ is continuous, it is bounded, so $f$ is bounded all over $Bbb R$ (why?). Also, $f:[0,2]toBbb R$ is uniformly continuous, being continuous on a compact set. Thus, given $varepsilon >0$ there exists $delta>0$ such that, whenever $|x-y|<delta,x,yin[0,2]$, then $|f(x)-f(y)|<varepsilon$. Pick arbitrary $x,y$, and assume $x<y$ with $|x-y|<delta$. We may take $delta <1$. I claim there is an integer $n$ such that $x-n,y-nin [0,2]$. Then $|x-y|=|x-n-(y-n)|$ and $f(x-n)=f(x)$, $f(y-n)=f(y)$. Can you continue now?
Drawing a picture would prove useful. Essentially, you're translating the problem to $[0,2]$ where we already solved the issue.
$endgroup$
$begingroup$
ah, I was trying to deal with the case where, say, we have $x= 0.99$ and $y = 1.01$. Considering $f$ on $[0,2]$ instead of $[0,1]$ is a smart way to deal with that!
$endgroup$
– MCT
Mar 4 '15 at 22:06
add a comment |
$begingroup$
Suppose that $f$ has period one. Since $f:[0,2]toBbb R$ is continuous, it is bounded, so $f$ is bounded all over $Bbb R$ (why?). Also, $f:[0,2]toBbb R$ is uniformly continuous, being continuous on a compact set. Thus, given $varepsilon >0$ there exists $delta>0$ such that, whenever $|x-y|<delta,x,yin[0,2]$, then $|f(x)-f(y)|<varepsilon$. Pick arbitrary $x,y$, and assume $x<y$ with $|x-y|<delta$. We may take $delta <1$. I claim there is an integer $n$ such that $x-n,y-nin [0,2]$. Then $|x-y|=|x-n-(y-n)|$ and $f(x-n)=f(x)$, $f(y-n)=f(y)$. Can you continue now?
Drawing a picture would prove useful. Essentially, you're translating the problem to $[0,2]$ where we already solved the issue.
$endgroup$
$begingroup$
ah, I was trying to deal with the case where, say, we have $x= 0.99$ and $y = 1.01$. Considering $f$ on $[0,2]$ instead of $[0,1]$ is a smart way to deal with that!
$endgroup$
– MCT
Mar 4 '15 at 22:06
add a comment |
$begingroup$
Suppose that $f$ has period one. Since $f:[0,2]toBbb R$ is continuous, it is bounded, so $f$ is bounded all over $Bbb R$ (why?). Also, $f:[0,2]toBbb R$ is uniformly continuous, being continuous on a compact set. Thus, given $varepsilon >0$ there exists $delta>0$ such that, whenever $|x-y|<delta,x,yin[0,2]$, then $|f(x)-f(y)|<varepsilon$. Pick arbitrary $x,y$, and assume $x<y$ with $|x-y|<delta$. We may take $delta <1$. I claim there is an integer $n$ such that $x-n,y-nin [0,2]$. Then $|x-y|=|x-n-(y-n)|$ and $f(x-n)=f(x)$, $f(y-n)=f(y)$. Can you continue now?
Drawing a picture would prove useful. Essentially, you're translating the problem to $[0,2]$ where we already solved the issue.
$endgroup$
Suppose that $f$ has period one. Since $f:[0,2]toBbb R$ is continuous, it is bounded, so $f$ is bounded all over $Bbb R$ (why?). Also, $f:[0,2]toBbb R$ is uniformly continuous, being continuous on a compact set. Thus, given $varepsilon >0$ there exists $delta>0$ such that, whenever $|x-y|<delta,x,yin[0,2]$, then $|f(x)-f(y)|<varepsilon$. Pick arbitrary $x,y$, and assume $x<y$ with $|x-y|<delta$. We may take $delta <1$. I claim there is an integer $n$ such that $x-n,y-nin [0,2]$. Then $|x-y|=|x-n-(y-n)|$ and $f(x-n)=f(x)$, $f(y-n)=f(y)$. Can you continue now?
Drawing a picture would prove useful. Essentially, you're translating the problem to $[0,2]$ where we already solved the issue.
edited Mar 4 '15 at 22:13
answered Apr 30 '14 at 1:19


Pedro Tamaroff♦Pedro Tamaroff
96.7k10153297
96.7k10153297
$begingroup$
ah, I was trying to deal with the case where, say, we have $x= 0.99$ and $y = 1.01$. Considering $f$ on $[0,2]$ instead of $[0,1]$ is a smart way to deal with that!
$endgroup$
– MCT
Mar 4 '15 at 22:06
add a comment |
$begingroup$
ah, I was trying to deal with the case where, say, we have $x= 0.99$ and $y = 1.01$. Considering $f$ on $[0,2]$ instead of $[0,1]$ is a smart way to deal with that!
$endgroup$
– MCT
Mar 4 '15 at 22:06
$begingroup$
ah, I was trying to deal with the case where, say, we have $x= 0.99$ and $y = 1.01$. Considering $f$ on $[0,2]$ instead of $[0,1]$ is a smart way to deal with that!
$endgroup$
– MCT
Mar 4 '15 at 22:06
$begingroup$
ah, I was trying to deal with the case where, say, we have $x= 0.99$ and $y = 1.01$. Considering $f$ on $[0,2]$ instead of $[0,1]$ is a smart way to deal with that!
$endgroup$
– MCT
Mar 4 '15 at 22:06
add a comment |
$begingroup$
A periodic continuous function is simply a function defined on a circle,
$$f:S^1 longrightarrow R.$$
Circle is a compact space. A continuous function on a compact space is both bounded and uniformly continuous.
$endgroup$
$begingroup$
But $f$ is defined on $mathbb R$, not on the circle.
$endgroup$
– Al Jebr
Jun 24 '18 at 18:54
$begingroup$
AI Jebr: We can identify a periodic function with one defined on a circle. You could also argue by noting that the image of a periodic function is the same as its image when restricted to one cycle.
$endgroup$
– Behnam Esmayli
Jun 24 '18 at 22:38
add a comment |
$begingroup$
A periodic continuous function is simply a function defined on a circle,
$$f:S^1 longrightarrow R.$$
Circle is a compact space. A continuous function on a compact space is both bounded and uniformly continuous.
$endgroup$
$begingroup$
But $f$ is defined on $mathbb R$, not on the circle.
$endgroup$
– Al Jebr
Jun 24 '18 at 18:54
$begingroup$
AI Jebr: We can identify a periodic function with one defined on a circle. You could also argue by noting that the image of a periodic function is the same as its image when restricted to one cycle.
$endgroup$
– Behnam Esmayli
Jun 24 '18 at 22:38
add a comment |
$begingroup$
A periodic continuous function is simply a function defined on a circle,
$$f:S^1 longrightarrow R.$$
Circle is a compact space. A continuous function on a compact space is both bounded and uniformly continuous.
$endgroup$
A periodic continuous function is simply a function defined on a circle,
$$f:S^1 longrightarrow R.$$
Circle is a compact space. A continuous function on a compact space is both bounded and uniformly continuous.
answered Mar 9 '17 at 6:12


Behnam EsmayliBehnam Esmayli
1,956515
1,956515
$begingroup$
But $f$ is defined on $mathbb R$, not on the circle.
$endgroup$
– Al Jebr
Jun 24 '18 at 18:54
$begingroup$
AI Jebr: We can identify a periodic function with one defined on a circle. You could also argue by noting that the image of a periodic function is the same as its image when restricted to one cycle.
$endgroup$
– Behnam Esmayli
Jun 24 '18 at 22:38
add a comment |
$begingroup$
But $f$ is defined on $mathbb R$, not on the circle.
$endgroup$
– Al Jebr
Jun 24 '18 at 18:54
$begingroup$
AI Jebr: We can identify a periodic function with one defined on a circle. You could also argue by noting that the image of a periodic function is the same as its image when restricted to one cycle.
$endgroup$
– Behnam Esmayli
Jun 24 '18 at 22:38
$begingroup$
But $f$ is defined on $mathbb R$, not on the circle.
$endgroup$
– Al Jebr
Jun 24 '18 at 18:54
$begingroup$
But $f$ is defined on $mathbb R$, not on the circle.
$endgroup$
– Al Jebr
Jun 24 '18 at 18:54
$begingroup$
AI Jebr: We can identify a periodic function with one defined on a circle. You could also argue by noting that the image of a periodic function is the same as its image when restricted to one cycle.
$endgroup$
– Behnam Esmayli
Jun 24 '18 at 22:38
$begingroup$
AI Jebr: We can identify a periodic function with one defined on a circle. You could also argue by noting that the image of a periodic function is the same as its image when restricted to one cycle.
$endgroup$
– Behnam Esmayli
Jun 24 '18 at 22:38
add a comment |
$begingroup$
Here's proof for boundedness part by contradiction :
Fix $a in Bbb R$. Consider the domain $I=[a,a+p]$. Assume $f$ is not bounded. Then there exists a sequence $x_n in I$ such that $|f(x_n)| gt n$.
Since $x_n$ is bounded and consists of real numbers, it has a convergent subsequence $x_{n_k}$. But $I$ is closed hence $x_{n_k}$ converges inside $I$. i.e. $exists x in I$ such that $x_{n_k} to x$.
What we have got is a sequence $x_{n_k} to x in I$ and $|f(x_{n_k})| to infty ; (because |f(x_{n_k})| gt n_k)$. This is contradiction for $f$ being continuous on $I$ and hence at $x$.
$endgroup$
add a comment |
$begingroup$
Here's proof for boundedness part by contradiction :
Fix $a in Bbb R$. Consider the domain $I=[a,a+p]$. Assume $f$ is not bounded. Then there exists a sequence $x_n in I$ such that $|f(x_n)| gt n$.
Since $x_n$ is bounded and consists of real numbers, it has a convergent subsequence $x_{n_k}$. But $I$ is closed hence $x_{n_k}$ converges inside $I$. i.e. $exists x in I$ such that $x_{n_k} to x$.
What we have got is a sequence $x_{n_k} to x in I$ and $|f(x_{n_k})| to infty ; (because |f(x_{n_k})| gt n_k)$. This is contradiction for $f$ being continuous on $I$ and hence at $x$.
$endgroup$
add a comment |
$begingroup$
Here's proof for boundedness part by contradiction :
Fix $a in Bbb R$. Consider the domain $I=[a,a+p]$. Assume $f$ is not bounded. Then there exists a sequence $x_n in I$ such that $|f(x_n)| gt n$.
Since $x_n$ is bounded and consists of real numbers, it has a convergent subsequence $x_{n_k}$. But $I$ is closed hence $x_{n_k}$ converges inside $I$. i.e. $exists x in I$ such that $x_{n_k} to x$.
What we have got is a sequence $x_{n_k} to x in I$ and $|f(x_{n_k})| to infty ; (because |f(x_{n_k})| gt n_k)$. This is contradiction for $f$ being continuous on $I$ and hence at $x$.
$endgroup$
Here's proof for boundedness part by contradiction :
Fix $a in Bbb R$. Consider the domain $I=[a,a+p]$. Assume $f$ is not bounded. Then there exists a sequence $x_n in I$ such that $|f(x_n)| gt n$.
Since $x_n$ is bounded and consists of real numbers, it has a convergent subsequence $x_{n_k}$. But $I$ is closed hence $x_{n_k}$ converges inside $I$. i.e. $exists x in I$ such that $x_{n_k} to x$.
What we have got is a sequence $x_{n_k} to x in I$ and $|f(x_{n_k})| to infty ; (because |f(x_{n_k})| gt n_k)$. This is contradiction for $f$ being continuous on $I$ and hence at $x$.
answered Mar 9 '17 at 5:22
Error 404Error 404
3,84321336
3,84321336
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