How to prove that $g(x)equiv f(x)$?
Clash Royale CLAN TAG#URR8PPP
up vote
2
down vote
favorite
Suppose that $f,g$ are periodic functions, and $limlimits_xto+infty [f(x)-g(x)]=0$, prove that $f(x)equiv g(x)$.
I've solved it if we add the condition that $f(x)$ is continuous.
But the continuity condition is un-needed.
==============
additional remarks =================
I know the fact that
$limlimits_xto+infty t(x)=0$ implies $forall x_nsubset Dom(t)$, if $limlimits_ntoinfty x_n=+infty$, then $limlimits_ntoinfty t(x_n)=0$,
which is called Heine Theorem in my textbook.
But I don't think you can put two different sequence here even both of them turn to infinity.
calculus
add a comment |Â
up vote
2
down vote
favorite
Suppose that $f,g$ are periodic functions, and $limlimits_xto+infty [f(x)-g(x)]=0$, prove that $f(x)equiv g(x)$.
I've solved it if we add the condition that $f(x)$ is continuous.
But the continuity condition is un-needed.
==============
additional remarks =================
I know the fact that
$limlimits_xto+infty t(x)=0$ implies $forall x_nsubset Dom(t)$, if $limlimits_ntoinfty x_n=+infty$, then $limlimits_ntoinfty t(x_n)=0$,
which is called Heine Theorem in my textbook.
But I don't think you can put two different sequence here even both of them turn to infinity.
calculus
add a comment |Â
up vote
2
down vote
favorite
up vote
2
down vote
favorite
Suppose that $f,g$ are periodic functions, and $limlimits_xto+infty [f(x)-g(x)]=0$, prove that $f(x)equiv g(x)$.
I've solved it if we add the condition that $f(x)$ is continuous.
But the continuity condition is un-needed.
==============
additional remarks =================
I know the fact that
$limlimits_xto+infty t(x)=0$ implies $forall x_nsubset Dom(t)$, if $limlimits_ntoinfty x_n=+infty$, then $limlimits_ntoinfty t(x_n)=0$,
which is called Heine Theorem in my textbook.
But I don't think you can put two different sequence here even both of them turn to infinity.
calculus
Suppose that $f,g$ are periodic functions, and $limlimits_xto+infty [f(x)-g(x)]=0$, prove that $f(x)equiv g(x)$.
I've solved it if we add the condition that $f(x)$ is continuous.
But the continuity condition is un-needed.
==============
additional remarks =================
I know the fact that
$limlimits_xto+infty t(x)=0$ implies $forall x_nsubset Dom(t)$, if $limlimits_ntoinfty x_n=+infty$, then $limlimits_ntoinfty t(x_n)=0$,
which is called Heine Theorem in my textbook.
But I don't think you can put two different sequence here even both of them turn to infinity.
calculus
edited Jul 17 at 16:44
asked Jul 17 at 4:13


闫嘉ç¦
41219
41219
add a comment |Â
add a comment |Â
1 Answer
1
active
oldest
votes
up vote
6
down vote
The idea is:
Let $p_1,p_2$ the periods of $f$ and $g$ respectively (suppose that are positive, in another case rewrite the proof). Now we have that $f(x + np_1) = f(x)$ and $g(x + np_2)= g(x)$ for all $n in mathbbN$. From hypothesis, for each fixed $y in mathbbR$ we have $lim_n to +infty [f(y+ np_1) - g(y + np_2)] = 0$. By definition of limit in sequences, the last means that for every $epsilon > 0$ there exists a great $n$ such that
$$|f(y) - g(y)| = |f(y + np_1) - g(y + np_2)| < epsilon$$
Then $|f(y) - g(y)|=0$, and this implies $f(y) = g(y)$.
The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ï¿¥ã€Â$y+mp_1=y+np_2$.
– é—«å˜‰ç¦
Jul 17 at 5:07
@闫嘉ç¦ The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
– Hanul Jeon
Jul 17 at 6:00
@HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
– é—«å˜‰ç¦
Jul 17 at 6:22
1
@闫嘉ç¦ Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
– Holo
Jul 17 at 10:21
2
Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
– Peter Szilas
Jul 17 at 18:25
 |Â
show 5 more comments
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
6
down vote
The idea is:
Let $p_1,p_2$ the periods of $f$ and $g$ respectively (suppose that are positive, in another case rewrite the proof). Now we have that $f(x + np_1) = f(x)$ and $g(x + np_2)= g(x)$ for all $n in mathbbN$. From hypothesis, for each fixed $y in mathbbR$ we have $lim_n to +infty [f(y+ np_1) - g(y + np_2)] = 0$. By definition of limit in sequences, the last means that for every $epsilon > 0$ there exists a great $n$ such that
$$|f(y) - g(y)| = |f(y + np_1) - g(y + np_2)| < epsilon$$
Then $|f(y) - g(y)|=0$, and this implies $f(y) = g(y)$.
The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ï¿¥ã€Â$y+mp_1=y+np_2$.
– é—«å˜‰ç¦
Jul 17 at 5:07
@闫嘉ç¦ The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
– Hanul Jeon
Jul 17 at 6:00
@HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
– é—«å˜‰ç¦
Jul 17 at 6:22
1
@闫嘉ç¦ Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
– Holo
Jul 17 at 10:21
2
Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
– Peter Szilas
Jul 17 at 18:25
 |Â
show 5 more comments
up vote
6
down vote
The idea is:
Let $p_1,p_2$ the periods of $f$ and $g$ respectively (suppose that are positive, in another case rewrite the proof). Now we have that $f(x + np_1) = f(x)$ and $g(x + np_2)= g(x)$ for all $n in mathbbN$. From hypothesis, for each fixed $y in mathbbR$ we have $lim_n to +infty [f(y+ np_1) - g(y + np_2)] = 0$. By definition of limit in sequences, the last means that for every $epsilon > 0$ there exists a great $n$ such that
$$|f(y) - g(y)| = |f(y + np_1) - g(y + np_2)| < epsilon$$
Then $|f(y) - g(y)|=0$, and this implies $f(y) = g(y)$.
The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ï¿¥ã€Â$y+mp_1=y+np_2$.
– é—«å˜‰ç¦
Jul 17 at 5:07
@闫嘉ç¦ The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
– Hanul Jeon
Jul 17 at 6:00
@HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
– é—«å˜‰ç¦
Jul 17 at 6:22
1
@闫嘉ç¦ Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
– Holo
Jul 17 at 10:21
2
Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
– Peter Szilas
Jul 17 at 18:25
 |Â
show 5 more comments
up vote
6
down vote
up vote
6
down vote
The idea is:
Let $p_1,p_2$ the periods of $f$ and $g$ respectively (suppose that are positive, in another case rewrite the proof). Now we have that $f(x + np_1) = f(x)$ and $g(x + np_2)= g(x)$ for all $n in mathbbN$. From hypothesis, for each fixed $y in mathbbR$ we have $lim_n to +infty [f(y+ np_1) - g(y + np_2)] = 0$. By definition of limit in sequences, the last means that for every $epsilon > 0$ there exists a great $n$ such that
$$|f(y) - g(y)| = |f(y + np_1) - g(y + np_2)| < epsilon$$
Then $|f(y) - g(y)|=0$, and this implies $f(y) = g(y)$.
The idea is:
Let $p_1,p_2$ the periods of $f$ and $g$ respectively (suppose that are positive, in another case rewrite the proof). Now we have that $f(x + np_1) = f(x)$ and $g(x + np_2)= g(x)$ for all $n in mathbbN$. From hypothesis, for each fixed $y in mathbbR$ we have $lim_n to +infty [f(y+ np_1) - g(y + np_2)] = 0$. By definition of limit in sequences, the last means that for every $epsilon > 0$ there exists a great $n$ such that
$$|f(y) - g(y)| = |f(y + np_1) - g(y + np_2)| < epsilon$$
Then $|f(y) - g(y)|=0$, and this implies $f(y) = g(y)$.
edited Jul 17 at 4:36
answered Jul 17 at 4:28
Inzit
935412
935412
The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ï¿¥ã€Â$y+mp_1=y+np_2$.
– é—«å˜‰ç¦
Jul 17 at 5:07
@闫嘉ç¦ The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
– Hanul Jeon
Jul 17 at 6:00
@HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
– é—«å˜‰ç¦
Jul 17 at 6:22
1
@闫嘉ç¦ Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
– Holo
Jul 17 at 10:21
2
Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
– Peter Szilas
Jul 17 at 18:25
 |Â
show 5 more comments
The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ï¿¥ã€Â$y+mp_1=y+np_2$.
– é—«å˜‰ç¦
Jul 17 at 5:07
@闫嘉ç¦ The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
– Hanul Jeon
Jul 17 at 6:00
@HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
– é—«å˜‰ç¦
Jul 17 at 6:22
1
@闫嘉ç¦ Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
– Holo
Jul 17 at 10:21
2
Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
– Peter Szilas
Jul 17 at 18:25
The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ï¿¥ã€Â$y+mp_1=y+np_2$.
– é—«å˜‰ç¦
Jul 17 at 5:07
The $<$ doesn't hold becase generally $y+np_1=y+np_2$. And if $dfracp_1 p_2$ is not rational, we cannot find any $m,ninmathbfN_+$ such that ï¿¥ã€Â$y+mp_1=y+np_2$.
– é—«å˜‰ç¦
Jul 17 at 5:07
@闫嘉ç¦ The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
– Hanul Jeon
Jul 17 at 6:00
@闫嘉ç¦ The answer does not require $y+mp_1$ and $y+np_2$ are same. The answer just uses the definition of limit.
– Hanul Jeon
Jul 17 at 6:00
@HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
– é—«å˜‰ç¦
Jul 17 at 6:22
@HanulJeon Acutually I don't understand why $limlimits_ntoinfty [f(y+np_1)-g(y+np_2)]=0$ holds
– é—«å˜‰ç¦
Jul 17 at 6:22
1
1
@闫嘉ç¦ Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
– Holo
Jul 17 at 10:21
@闫嘉ç¦ Inzit use the fact that $lim_xtoinftyf(x)$ exists if and only if every sequence $(x_n)_nin Bbb N$ if for all $n$ we have $x_ninmboxDom(f)$ and $lim_ntoinfty x_n=infty$ then $lim_ntoinftyf(x_n)=lim_xtoinftyf(x)$, now define $h=f-g$ and use this fact to see how this limit holds
– Holo
Jul 17 at 10:21
2
2
Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
– Peter Szilas
Jul 17 at 18:25
Oops, seems I missed something reading the OP's answer.As you discussed already the OP is comparing 2 different points:$ y_1= y+np_1 not = y +np_2=y_2$.Taking limit as n goes to $infty$ one cannot use limit $ |f(y)-g(y)| $ as y goes to $infty$(same point y).If one has $n_1p_1=n_2p_2 =T$ as common period T, then comparing $|f(y+nT)-g(y+nT)| $as n goes to $infty,$ for every y, seems fine.Otherwise I would not know.
– Peter Szilas
Jul 17 at 18:25
 |Â
show 5 more comments
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f2854126%2fhow-to-prove-that-gx-equiv-fx%23new-answer', 'question_page');
);
Post as a guest
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password