Show uniform convergence of $f_n (x)=sqrt {sin left(frac {x}{n}right)+cosleft(frac {x}{n}right) }$












0












$begingroup$


How can I prove uniform convergence of
$$
f_n (x)
=sqrt {sin left(frac {x}{n}right)+cosleft(frac {x}{n}right)}
$$

for $x in left[0,frac{pi}{2}right]$



I have been able to show its pointwise limit is $1$.










share|cite|improve this question











$endgroup$












  • $begingroup$
    Have have you tried? How did you show the pointwise limit?
    $endgroup$
    – Viktor Glombik
    Jan 26 at 14:16










  • $begingroup$
    @ViktorGlombik, for any fixed $x $, by the continuity of the square root, sine and cosine functions, as n approaches infinity, we have $sqrt (1)=1$
    $endgroup$
    – user397197
    Jan 26 at 14:19
















0












$begingroup$


How can I prove uniform convergence of
$$
f_n (x)
=sqrt {sin left(frac {x}{n}right)+cosleft(frac {x}{n}right)}
$$

for $x in left[0,frac{pi}{2}right]$



I have been able to show its pointwise limit is $1$.










share|cite|improve this question











$endgroup$












  • $begingroup$
    Have have you tried? How did you show the pointwise limit?
    $endgroup$
    – Viktor Glombik
    Jan 26 at 14:16










  • $begingroup$
    @ViktorGlombik, for any fixed $x $, by the continuity of the square root, sine and cosine functions, as n approaches infinity, we have $sqrt (1)=1$
    $endgroup$
    – user397197
    Jan 26 at 14:19














0












0








0





$begingroup$


How can I prove uniform convergence of
$$
f_n (x)
=sqrt {sin left(frac {x}{n}right)+cosleft(frac {x}{n}right)}
$$

for $x in left[0,frac{pi}{2}right]$



I have been able to show its pointwise limit is $1$.










share|cite|improve this question











$endgroup$




How can I prove uniform convergence of
$$
f_n (x)
=sqrt {sin left(frac {x}{n}right)+cosleft(frac {x}{n}right)}
$$

for $x in left[0,frac{pi}{2}right]$



I have been able to show its pointwise limit is $1$.







real-analysis analysis convergence uniform-convergence






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Jan 26 at 14:58









Viktor Glombik

1,2032528




1,2032528










asked Jan 26 at 14:10







user397197



















  • $begingroup$
    Have have you tried? How did you show the pointwise limit?
    $endgroup$
    – Viktor Glombik
    Jan 26 at 14:16










  • $begingroup$
    @ViktorGlombik, for any fixed $x $, by the continuity of the square root, sine and cosine functions, as n approaches infinity, we have $sqrt (1)=1$
    $endgroup$
    – user397197
    Jan 26 at 14:19


















  • $begingroup$
    Have have you tried? How did you show the pointwise limit?
    $endgroup$
    – Viktor Glombik
    Jan 26 at 14:16










  • $begingroup$
    @ViktorGlombik, for any fixed $x $, by the continuity of the square root, sine and cosine functions, as n approaches infinity, we have $sqrt (1)=1$
    $endgroup$
    – user397197
    Jan 26 at 14:19
















$begingroup$
Have have you tried? How did you show the pointwise limit?
$endgroup$
– Viktor Glombik
Jan 26 at 14:16




$begingroup$
Have have you tried? How did you show the pointwise limit?
$endgroup$
– Viktor Glombik
Jan 26 at 14:16












$begingroup$
@ViktorGlombik, for any fixed $x $, by the continuity of the square root, sine and cosine functions, as n approaches infinity, we have $sqrt (1)=1$
$endgroup$
– user397197
Jan 26 at 14:19




$begingroup$
@ViktorGlombik, for any fixed $x $, by the continuity of the square root, sine and cosine functions, as n approaches infinity, we have $sqrt (1)=1$
$endgroup$
– user397197
Jan 26 at 14:19










2 Answers
2






active

oldest

votes


















2












$begingroup$

Hint: Define
$$f(x)= sqrt{sin(x) + cos(x)}$$
This function is continuous and thus uniformly continuous on $[0; frac{pi}{2}]$.



Added: We have that for every $xin [0; frac{pi}{2}]$ that
$$ leftvert frac{x}{n} - frac{x}{m} rightvert
= leftvert frac{1}{n} - frac{1}{m} rightvert cdot vert x vert
leq frac{pi}{2} leftvert frac{1}{n} - frac{1}{m} rightvert $$

Furthermore, we have
$$ vert f_n(x) - f_m(x) vert = vert f(frac{x}{n}) - f(frac{x}{m}) vert. $$
Of course, we don't need the uniform continuity (as shown in the comments by p4sch), but I consider it usefull to think this way. It allows one to also tackle different problems (like for example the uniform convergence of $g_n(x)=sqrt{x^2+frac{1}{n}}$ to the absolute value on the whole real line).






share|cite|improve this answer











$endgroup$













  • $begingroup$
    I'm not sure how this should help to show that $f_n$ congerves uniformly. I think of $f_n(x)=x^n$, which are uniformly continuous on $[0,1]$ as well while it doesn't converge uniformly on $[0,1]$.
    $endgroup$
    – Mundron Schmidt
    Jan 26 at 14:21










  • $begingroup$
    I know this but how does how does uniform continuity and uniform convergence relate in this case. Is there any theoren I am missing?
    $endgroup$
    – user397197
    Jan 26 at 14:24






  • 1




    $begingroup$
    You have $f_n(x) = f(x/n)$. Since $|f(0) -f(x)|< varepsilon$ for all $|x|< delta$, you get for all $1/n < delta$ and $x in [0,pi/2]$ that $|f_n(x)| < varepsilon$.
    $endgroup$
    – p4sch
    Jan 26 at 14:30












  • $begingroup$
    @MundronSchmidt Honestly I do not see how your example is related to what I wrote. I consider a single function which reproduces the family and is uniformly continuous, but in your example only every individual function is uniformly continuous. Maybe you could elaborate on your comment?
    $endgroup$
    – Severin Schraven
    Jan 26 at 14:51










  • $begingroup$
    @stackuser I did not add the second part so that you could try on your own. But it seems it was not clear enough. I guess now you should be able to fill in the gaps. Or do you want me to do it as well?
    $endgroup$
    – Severin Schraven
    Jan 26 at 14:53



















2












$begingroup$

Hint: use the inequalities $|sin(t)| leq |t|$ and $|cos(t)-1|leq frac{t^2}{2}$.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I am yet to know how to apply the hint. please guide me
    $endgroup$
    – user397197
    Jan 26 at 15:39













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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









2












$begingroup$

Hint: Define
$$f(x)= sqrt{sin(x) + cos(x)}$$
This function is continuous and thus uniformly continuous on $[0; frac{pi}{2}]$.



Added: We have that for every $xin [0; frac{pi}{2}]$ that
$$ leftvert frac{x}{n} - frac{x}{m} rightvert
= leftvert frac{1}{n} - frac{1}{m} rightvert cdot vert x vert
leq frac{pi}{2} leftvert frac{1}{n} - frac{1}{m} rightvert $$

Furthermore, we have
$$ vert f_n(x) - f_m(x) vert = vert f(frac{x}{n}) - f(frac{x}{m}) vert. $$
Of course, we don't need the uniform continuity (as shown in the comments by p4sch), but I consider it usefull to think this way. It allows one to also tackle different problems (like for example the uniform convergence of $g_n(x)=sqrt{x^2+frac{1}{n}}$ to the absolute value on the whole real line).






share|cite|improve this answer











$endgroup$













  • $begingroup$
    I'm not sure how this should help to show that $f_n$ congerves uniformly. I think of $f_n(x)=x^n$, which are uniformly continuous on $[0,1]$ as well while it doesn't converge uniformly on $[0,1]$.
    $endgroup$
    – Mundron Schmidt
    Jan 26 at 14:21










  • $begingroup$
    I know this but how does how does uniform continuity and uniform convergence relate in this case. Is there any theoren I am missing?
    $endgroup$
    – user397197
    Jan 26 at 14:24






  • 1




    $begingroup$
    You have $f_n(x) = f(x/n)$. Since $|f(0) -f(x)|< varepsilon$ for all $|x|< delta$, you get for all $1/n < delta$ and $x in [0,pi/2]$ that $|f_n(x)| < varepsilon$.
    $endgroup$
    – p4sch
    Jan 26 at 14:30












  • $begingroup$
    @MundronSchmidt Honestly I do not see how your example is related to what I wrote. I consider a single function which reproduces the family and is uniformly continuous, but in your example only every individual function is uniformly continuous. Maybe you could elaborate on your comment?
    $endgroup$
    – Severin Schraven
    Jan 26 at 14:51










  • $begingroup$
    @stackuser I did not add the second part so that you could try on your own. But it seems it was not clear enough. I guess now you should be able to fill in the gaps. Or do you want me to do it as well?
    $endgroup$
    – Severin Schraven
    Jan 26 at 14:53
















2












$begingroup$

Hint: Define
$$f(x)= sqrt{sin(x) + cos(x)}$$
This function is continuous and thus uniformly continuous on $[0; frac{pi}{2}]$.



Added: We have that for every $xin [0; frac{pi}{2}]$ that
$$ leftvert frac{x}{n} - frac{x}{m} rightvert
= leftvert frac{1}{n} - frac{1}{m} rightvert cdot vert x vert
leq frac{pi}{2} leftvert frac{1}{n} - frac{1}{m} rightvert $$

Furthermore, we have
$$ vert f_n(x) - f_m(x) vert = vert f(frac{x}{n}) - f(frac{x}{m}) vert. $$
Of course, we don't need the uniform continuity (as shown in the comments by p4sch), but I consider it usefull to think this way. It allows one to also tackle different problems (like for example the uniform convergence of $g_n(x)=sqrt{x^2+frac{1}{n}}$ to the absolute value on the whole real line).






share|cite|improve this answer











$endgroup$













  • $begingroup$
    I'm not sure how this should help to show that $f_n$ congerves uniformly. I think of $f_n(x)=x^n$, which are uniformly continuous on $[0,1]$ as well while it doesn't converge uniformly on $[0,1]$.
    $endgroup$
    – Mundron Schmidt
    Jan 26 at 14:21










  • $begingroup$
    I know this but how does how does uniform continuity and uniform convergence relate in this case. Is there any theoren I am missing?
    $endgroup$
    – user397197
    Jan 26 at 14:24






  • 1




    $begingroup$
    You have $f_n(x) = f(x/n)$. Since $|f(0) -f(x)|< varepsilon$ for all $|x|< delta$, you get for all $1/n < delta$ and $x in [0,pi/2]$ that $|f_n(x)| < varepsilon$.
    $endgroup$
    – p4sch
    Jan 26 at 14:30












  • $begingroup$
    @MundronSchmidt Honestly I do not see how your example is related to what I wrote. I consider a single function which reproduces the family and is uniformly continuous, but in your example only every individual function is uniformly continuous. Maybe you could elaborate on your comment?
    $endgroup$
    – Severin Schraven
    Jan 26 at 14:51










  • $begingroup$
    @stackuser I did not add the second part so that you could try on your own. But it seems it was not clear enough. I guess now you should be able to fill in the gaps. Or do you want me to do it as well?
    $endgroup$
    – Severin Schraven
    Jan 26 at 14:53














2












2








2





$begingroup$

Hint: Define
$$f(x)= sqrt{sin(x) + cos(x)}$$
This function is continuous and thus uniformly continuous on $[0; frac{pi}{2}]$.



Added: We have that for every $xin [0; frac{pi}{2}]$ that
$$ leftvert frac{x}{n} - frac{x}{m} rightvert
= leftvert frac{1}{n} - frac{1}{m} rightvert cdot vert x vert
leq frac{pi}{2} leftvert frac{1}{n} - frac{1}{m} rightvert $$

Furthermore, we have
$$ vert f_n(x) - f_m(x) vert = vert f(frac{x}{n}) - f(frac{x}{m}) vert. $$
Of course, we don't need the uniform continuity (as shown in the comments by p4sch), but I consider it usefull to think this way. It allows one to also tackle different problems (like for example the uniform convergence of $g_n(x)=sqrt{x^2+frac{1}{n}}$ to the absolute value on the whole real line).






share|cite|improve this answer











$endgroup$



Hint: Define
$$f(x)= sqrt{sin(x) + cos(x)}$$
This function is continuous and thus uniformly continuous on $[0; frac{pi}{2}]$.



Added: We have that for every $xin [0; frac{pi}{2}]$ that
$$ leftvert frac{x}{n} - frac{x}{m} rightvert
= leftvert frac{1}{n} - frac{1}{m} rightvert cdot vert x vert
leq frac{pi}{2} leftvert frac{1}{n} - frac{1}{m} rightvert $$

Furthermore, we have
$$ vert f_n(x) - f_m(x) vert = vert f(frac{x}{n}) - f(frac{x}{m}) vert. $$
Of course, we don't need the uniform continuity (as shown in the comments by p4sch), but I consider it usefull to think this way. It allows one to also tackle different problems (like for example the uniform convergence of $g_n(x)=sqrt{x^2+frac{1}{n}}$ to the absolute value on the whole real line).







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Jan 26 at 14:43

























answered Jan 26 at 14:18









Severin SchravenSeverin Schraven

6,4501935




6,4501935












  • $begingroup$
    I'm not sure how this should help to show that $f_n$ congerves uniformly. I think of $f_n(x)=x^n$, which are uniformly continuous on $[0,1]$ as well while it doesn't converge uniformly on $[0,1]$.
    $endgroup$
    – Mundron Schmidt
    Jan 26 at 14:21










  • $begingroup$
    I know this but how does how does uniform continuity and uniform convergence relate in this case. Is there any theoren I am missing?
    $endgroup$
    – user397197
    Jan 26 at 14:24






  • 1




    $begingroup$
    You have $f_n(x) = f(x/n)$. Since $|f(0) -f(x)|< varepsilon$ for all $|x|< delta$, you get for all $1/n < delta$ and $x in [0,pi/2]$ that $|f_n(x)| < varepsilon$.
    $endgroup$
    – p4sch
    Jan 26 at 14:30












  • $begingroup$
    @MundronSchmidt Honestly I do not see how your example is related to what I wrote. I consider a single function which reproduces the family and is uniformly continuous, but in your example only every individual function is uniformly continuous. Maybe you could elaborate on your comment?
    $endgroup$
    – Severin Schraven
    Jan 26 at 14:51










  • $begingroup$
    @stackuser I did not add the second part so that you could try on your own. But it seems it was not clear enough. I guess now you should be able to fill in the gaps. Or do you want me to do it as well?
    $endgroup$
    – Severin Schraven
    Jan 26 at 14:53


















  • $begingroup$
    I'm not sure how this should help to show that $f_n$ congerves uniformly. I think of $f_n(x)=x^n$, which are uniformly continuous on $[0,1]$ as well while it doesn't converge uniformly on $[0,1]$.
    $endgroup$
    – Mundron Schmidt
    Jan 26 at 14:21










  • $begingroup$
    I know this but how does how does uniform continuity and uniform convergence relate in this case. Is there any theoren I am missing?
    $endgroup$
    – user397197
    Jan 26 at 14:24






  • 1




    $begingroup$
    You have $f_n(x) = f(x/n)$. Since $|f(0) -f(x)|< varepsilon$ for all $|x|< delta$, you get for all $1/n < delta$ and $x in [0,pi/2]$ that $|f_n(x)| < varepsilon$.
    $endgroup$
    – p4sch
    Jan 26 at 14:30












  • $begingroup$
    @MundronSchmidt Honestly I do not see how your example is related to what I wrote. I consider a single function which reproduces the family and is uniformly continuous, but in your example only every individual function is uniformly continuous. Maybe you could elaborate on your comment?
    $endgroup$
    – Severin Schraven
    Jan 26 at 14:51










  • $begingroup$
    @stackuser I did not add the second part so that you could try on your own. But it seems it was not clear enough. I guess now you should be able to fill in the gaps. Or do you want me to do it as well?
    $endgroup$
    – Severin Schraven
    Jan 26 at 14:53
















$begingroup$
I'm not sure how this should help to show that $f_n$ congerves uniformly. I think of $f_n(x)=x^n$, which are uniformly continuous on $[0,1]$ as well while it doesn't converge uniformly on $[0,1]$.
$endgroup$
– Mundron Schmidt
Jan 26 at 14:21




$begingroup$
I'm not sure how this should help to show that $f_n$ congerves uniformly. I think of $f_n(x)=x^n$, which are uniformly continuous on $[0,1]$ as well while it doesn't converge uniformly on $[0,1]$.
$endgroup$
– Mundron Schmidt
Jan 26 at 14:21












$begingroup$
I know this but how does how does uniform continuity and uniform convergence relate in this case. Is there any theoren I am missing?
$endgroup$
– user397197
Jan 26 at 14:24




$begingroup$
I know this but how does how does uniform continuity and uniform convergence relate in this case. Is there any theoren I am missing?
$endgroup$
– user397197
Jan 26 at 14:24




1




1




$begingroup$
You have $f_n(x) = f(x/n)$. Since $|f(0) -f(x)|< varepsilon$ for all $|x|< delta$, you get for all $1/n < delta$ and $x in [0,pi/2]$ that $|f_n(x)| < varepsilon$.
$endgroup$
– p4sch
Jan 26 at 14:30






$begingroup$
You have $f_n(x) = f(x/n)$. Since $|f(0) -f(x)|< varepsilon$ for all $|x|< delta$, you get for all $1/n < delta$ and $x in [0,pi/2]$ that $|f_n(x)| < varepsilon$.
$endgroup$
– p4sch
Jan 26 at 14:30














$begingroup$
@MundronSchmidt Honestly I do not see how your example is related to what I wrote. I consider a single function which reproduces the family and is uniformly continuous, but in your example only every individual function is uniformly continuous. Maybe you could elaborate on your comment?
$endgroup$
– Severin Schraven
Jan 26 at 14:51




$begingroup$
@MundronSchmidt Honestly I do not see how your example is related to what I wrote. I consider a single function which reproduces the family and is uniformly continuous, but in your example only every individual function is uniformly continuous. Maybe you could elaborate on your comment?
$endgroup$
– Severin Schraven
Jan 26 at 14:51












$begingroup$
@stackuser I did not add the second part so that you could try on your own. But it seems it was not clear enough. I guess now you should be able to fill in the gaps. Or do you want me to do it as well?
$endgroup$
– Severin Schraven
Jan 26 at 14:53




$begingroup$
@stackuser I did not add the second part so that you could try on your own. But it seems it was not clear enough. I guess now you should be able to fill in the gaps. Or do you want me to do it as well?
$endgroup$
– Severin Schraven
Jan 26 at 14:53











2












$begingroup$

Hint: use the inequalities $|sin(t)| leq |t|$ and $|cos(t)-1|leq frac{t^2}{2}$.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I am yet to know how to apply the hint. please guide me
    $endgroup$
    – user397197
    Jan 26 at 15:39


















2












$begingroup$

Hint: use the inequalities $|sin(t)| leq |t|$ and $|cos(t)-1|leq frac{t^2}{2}$.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    I am yet to know how to apply the hint. please guide me
    $endgroup$
    – user397197
    Jan 26 at 15:39
















2












2








2





$begingroup$

Hint: use the inequalities $|sin(t)| leq |t|$ and $|cos(t)-1|leq frac{t^2}{2}$.






share|cite|improve this answer









$endgroup$



Hint: use the inequalities $|sin(t)| leq |t|$ and $|cos(t)-1|leq frac{t^2}{2}$.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Jan 26 at 14:17









Ewan DelanoyEwan Delanoy

41.9k443104




41.9k443104












  • $begingroup$
    I am yet to know how to apply the hint. please guide me
    $endgroup$
    – user397197
    Jan 26 at 15:39




















  • $begingroup$
    I am yet to know how to apply the hint. please guide me
    $endgroup$
    – user397197
    Jan 26 at 15:39


















$begingroup$
I am yet to know how to apply the hint. please guide me
$endgroup$
– user397197
Jan 26 at 15:39






$begingroup$
I am yet to know how to apply the hint. please guide me
$endgroup$
– user397197
Jan 26 at 15:39




















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