Function satisfying $limlimits_{xto 0}(f(x)-f(2x))$ but doesn't have $limlimits_{xto 0}f(x)$
$begingroup$
I want to find a function that does not have a limit as $limlimits_{xto 0}f(x)$, but the following limit does exist: $limlimits_{xto 0}(f(x)-f(2x))$.
Any direction or hint would be appreciated.
calculus
$endgroup$
add a comment |
$begingroup$
I want to find a function that does not have a limit as $limlimits_{xto 0}f(x)$, but the following limit does exist: $limlimits_{xto 0}(f(x)-f(2x))$.
Any direction or hint would be appreciated.
calculus
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1
$begingroup$
I guess $f(x)=sgn(x)$ would do the job. That is $f$ equals $1$, $0$ or $-1$ when $x$ is positive, null or negative, respectively.
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– Alejandro Nasif Salum
Dec 25 '17 at 8:38
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Does $pminfty$ count as existence of the limit?
$endgroup$
– Shashi
Dec 25 '17 at 8:44
add a comment |
$begingroup$
I want to find a function that does not have a limit as $limlimits_{xto 0}f(x)$, but the following limit does exist: $limlimits_{xto 0}(f(x)-f(2x))$.
Any direction or hint would be appreciated.
calculus
$endgroup$
I want to find a function that does not have a limit as $limlimits_{xto 0}f(x)$, but the following limit does exist: $limlimits_{xto 0}(f(x)-f(2x))$.
Any direction or hint would be appreciated.
calculus
calculus
edited Dec 21 '18 at 8:58
Saad
20.2k92352
20.2k92352
asked Dec 25 '17 at 8:34
Lin TanakaLin Tanaka
265
265
1
$begingroup$
I guess $f(x)=sgn(x)$ would do the job. That is $f$ equals $1$, $0$ or $-1$ when $x$ is positive, null or negative, respectively.
$endgroup$
– Alejandro Nasif Salum
Dec 25 '17 at 8:38
$begingroup$
Does $pminfty$ count as existence of the limit?
$endgroup$
– Shashi
Dec 25 '17 at 8:44
add a comment |
1
$begingroup$
I guess $f(x)=sgn(x)$ would do the job. That is $f$ equals $1$, $0$ or $-1$ when $x$ is positive, null or negative, respectively.
$endgroup$
– Alejandro Nasif Salum
Dec 25 '17 at 8:38
$begingroup$
Does $pminfty$ count as existence of the limit?
$endgroup$
– Shashi
Dec 25 '17 at 8:44
1
1
$begingroup$
I guess $f(x)=sgn(x)$ would do the job. That is $f$ equals $1$, $0$ or $-1$ when $x$ is positive, null or negative, respectively.
$endgroup$
– Alejandro Nasif Salum
Dec 25 '17 at 8:38
$begingroup$
I guess $f(x)=sgn(x)$ would do the job. That is $f$ equals $1$, $0$ or $-1$ when $x$ is positive, null or negative, respectively.
$endgroup$
– Alejandro Nasif Salum
Dec 25 '17 at 8:38
$begingroup$
Does $pminfty$ count as existence of the limit?
$endgroup$
– Shashi
Dec 25 '17 at 8:44
$begingroup$
Does $pminfty$ count as existence of the limit?
$endgroup$
– Shashi
Dec 25 '17 at 8:44
add a comment |
5 Answers
5
active
oldest
votes
$begingroup$
$f(x)=chi_{{bf{Q}}}(x)$, then $f(2x)=chi_{{bf{Q}}}(2x)=chi_{{bf{Q}}}(x)=f(x)$ but $lim_{xrightarrow 0}f(x)$ does not exist.
$endgroup$
$begingroup$
What is $chi_{{bf{Q}}}$?
$endgroup$
– Holo
Dec 25 '17 at 11:12
$begingroup$
The function takes the value $1$ for all rational numbers and $0$ otherwise.
$endgroup$
– user284331
Dec 25 '17 at 18:06
add a comment |
$begingroup$
Take
$f(x)=ln(x)$
$f(2x)=ln(2x)=ln(2)+ln(x)$
$g(x)=f(x)-f(2x)=-ln(2)$
Here $lim_{xto0}f(x)$ is undefined but $f(x)-f(2x)$ is a constant
$endgroup$
$begingroup$
Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
$endgroup$
– Holo
Dec 26 '17 at 7:47
add a comment |
$begingroup$
Hint. Take a function such that $$f(x)-f(2x)=fleft(frac{x}{2x} right)=fleft(frac 1 2 right)$$ You already know one of such functions I hope.
$endgroup$
add a comment |
$begingroup$
Any function such that $f(x)=f(2x)$ will work. Also if they differ in a constant, or if they differ in a function which is continuous at $x=0$ (but of course, that's almost rephrasing your question).
$endgroup$
add a comment |
$begingroup$
Here's an example that is rather continuous.
Take $f(x) = sinleft(2pifrac{ln |x|}{ln 2}right)$ defined for $x neq 0$ (you can set $f(0)=0$ if you want $f$ to be definied for all $x in mathbb R$). Then
$$f(2x) = sinleft(2pifrac{ln |2x|}{ln 2}right) = sinleft(2pifrac{ln2 + ln |x|}{ln 2}right) \= sinleft(2pi+2pifrac{ln |x|}{ln 2}right) = sinleft(2pifrac{ln |x|}{ln 2}right) \= f(x)$$
so
$$lim_{xto 0}(f(x)-f(2x)) = 0.$$
But $lim_{xto 0} f(x)$ is not defined since $f$ oscillates quickly as $x to 0$.
$endgroup$
add a comment |
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5 Answers
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5 Answers
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$begingroup$
$f(x)=chi_{{bf{Q}}}(x)$, then $f(2x)=chi_{{bf{Q}}}(2x)=chi_{{bf{Q}}}(x)=f(x)$ but $lim_{xrightarrow 0}f(x)$ does not exist.
$endgroup$
$begingroup$
What is $chi_{{bf{Q}}}$?
$endgroup$
– Holo
Dec 25 '17 at 11:12
$begingroup$
The function takes the value $1$ for all rational numbers and $0$ otherwise.
$endgroup$
– user284331
Dec 25 '17 at 18:06
add a comment |
$begingroup$
$f(x)=chi_{{bf{Q}}}(x)$, then $f(2x)=chi_{{bf{Q}}}(2x)=chi_{{bf{Q}}}(x)=f(x)$ but $lim_{xrightarrow 0}f(x)$ does not exist.
$endgroup$
$begingroup$
What is $chi_{{bf{Q}}}$?
$endgroup$
– Holo
Dec 25 '17 at 11:12
$begingroup$
The function takes the value $1$ for all rational numbers and $0$ otherwise.
$endgroup$
– user284331
Dec 25 '17 at 18:06
add a comment |
$begingroup$
$f(x)=chi_{{bf{Q}}}(x)$, then $f(2x)=chi_{{bf{Q}}}(2x)=chi_{{bf{Q}}}(x)=f(x)$ but $lim_{xrightarrow 0}f(x)$ does not exist.
$endgroup$
$f(x)=chi_{{bf{Q}}}(x)$, then $f(2x)=chi_{{bf{Q}}}(2x)=chi_{{bf{Q}}}(x)=f(x)$ but $lim_{xrightarrow 0}f(x)$ does not exist.
answered Dec 25 '17 at 8:39
user284331user284331
35.4k31646
35.4k31646
$begingroup$
What is $chi_{{bf{Q}}}$?
$endgroup$
– Holo
Dec 25 '17 at 11:12
$begingroup$
The function takes the value $1$ for all rational numbers and $0$ otherwise.
$endgroup$
– user284331
Dec 25 '17 at 18:06
add a comment |
$begingroup$
What is $chi_{{bf{Q}}}$?
$endgroup$
– Holo
Dec 25 '17 at 11:12
$begingroup$
The function takes the value $1$ for all rational numbers and $0$ otherwise.
$endgroup$
– user284331
Dec 25 '17 at 18:06
$begingroup$
What is $chi_{{bf{Q}}}$?
$endgroup$
– Holo
Dec 25 '17 at 11:12
$begingroup$
What is $chi_{{bf{Q}}}$?
$endgroup$
– Holo
Dec 25 '17 at 11:12
$begingroup$
The function takes the value $1$ for all rational numbers and $0$ otherwise.
$endgroup$
– user284331
Dec 25 '17 at 18:06
$begingroup$
The function takes the value $1$ for all rational numbers and $0$ otherwise.
$endgroup$
– user284331
Dec 25 '17 at 18:06
add a comment |
$begingroup$
Take
$f(x)=ln(x)$
$f(2x)=ln(2x)=ln(2)+ln(x)$
$g(x)=f(x)-f(2x)=-ln(2)$
Here $lim_{xto0}f(x)$ is undefined but $f(x)-f(2x)$ is a constant
$endgroup$
$begingroup$
Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
$endgroup$
– Holo
Dec 26 '17 at 7:47
add a comment |
$begingroup$
Take
$f(x)=ln(x)$
$f(2x)=ln(2x)=ln(2)+ln(x)$
$g(x)=f(x)-f(2x)=-ln(2)$
Here $lim_{xto0}f(x)$ is undefined but $f(x)-f(2x)$ is a constant
$endgroup$
$begingroup$
Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
$endgroup$
– Holo
Dec 26 '17 at 7:47
add a comment |
$begingroup$
Take
$f(x)=ln(x)$
$f(2x)=ln(2x)=ln(2)+ln(x)$
$g(x)=f(x)-f(2x)=-ln(2)$
Here $lim_{xto0}f(x)$ is undefined but $f(x)-f(2x)$ is a constant
$endgroup$
Take
$f(x)=ln(x)$
$f(2x)=ln(2x)=ln(2)+ln(x)$
$g(x)=f(x)-f(2x)=-ln(2)$
Here $lim_{xto0}f(x)$ is undefined but $f(x)-f(2x)$ is a constant
edited Dec 25 '17 at 9:44
answered Dec 25 '17 at 9:36
HoloHolo
6,08421131
6,08421131
$begingroup$
Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
$endgroup$
– Holo
Dec 26 '17 at 7:47
add a comment |
$begingroup$
Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
$endgroup$
– Holo
Dec 26 '17 at 7:47
$begingroup$
Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
$endgroup$
– Holo
Dec 26 '17 at 7:47
$begingroup$
Btw, if you consider $pminfty$ to be a limit then consider $f(x)=Im(ln(ix))=arg(ix)=arg(i2x)=Im(ln(i2x))=f(2x)implies f(x)-f(2x)=0forall x$ and $lim_{xto0^+}f(x)=pi/2,lim_{xto0^-}f(x)=-pi/2$
$endgroup$
– Holo
Dec 26 '17 at 7:47
add a comment |
$begingroup$
Hint. Take a function such that $$f(x)-f(2x)=fleft(frac{x}{2x} right)=fleft(frac 1 2 right)$$ You already know one of such functions I hope.
$endgroup$
add a comment |
$begingroup$
Hint. Take a function such that $$f(x)-f(2x)=fleft(frac{x}{2x} right)=fleft(frac 1 2 right)$$ You already know one of such functions I hope.
$endgroup$
add a comment |
$begingroup$
Hint. Take a function such that $$f(x)-f(2x)=fleft(frac{x}{2x} right)=fleft(frac 1 2 right)$$ You already know one of such functions I hope.
$endgroup$
Hint. Take a function such that $$f(x)-f(2x)=fleft(frac{x}{2x} right)=fleft(frac 1 2 right)$$ You already know one of such functions I hope.
answered Dec 25 '17 at 8:39
ShashiShashi
7,3151628
7,3151628
add a comment |
add a comment |
$begingroup$
Any function such that $f(x)=f(2x)$ will work. Also if they differ in a constant, or if they differ in a function which is continuous at $x=0$ (but of course, that's almost rephrasing your question).
$endgroup$
add a comment |
$begingroup$
Any function such that $f(x)=f(2x)$ will work. Also if they differ in a constant, or if they differ in a function which is continuous at $x=0$ (but of course, that's almost rephrasing your question).
$endgroup$
add a comment |
$begingroup$
Any function such that $f(x)=f(2x)$ will work. Also if they differ in a constant, or if they differ in a function which is continuous at $x=0$ (but of course, that's almost rephrasing your question).
$endgroup$
Any function such that $f(x)=f(2x)$ will work. Also if they differ in a constant, or if they differ in a function which is continuous at $x=0$ (but of course, that's almost rephrasing your question).
answered Dec 25 '17 at 8:42
Alejandro Nasif SalumAlejandro Nasif Salum
4,765118
4,765118
add a comment |
add a comment |
$begingroup$
Here's an example that is rather continuous.
Take $f(x) = sinleft(2pifrac{ln |x|}{ln 2}right)$ defined for $x neq 0$ (you can set $f(0)=0$ if you want $f$ to be definied for all $x in mathbb R$). Then
$$f(2x) = sinleft(2pifrac{ln |2x|}{ln 2}right) = sinleft(2pifrac{ln2 + ln |x|}{ln 2}right) \= sinleft(2pi+2pifrac{ln |x|}{ln 2}right) = sinleft(2pifrac{ln |x|}{ln 2}right) \= f(x)$$
so
$$lim_{xto 0}(f(x)-f(2x)) = 0.$$
But $lim_{xto 0} f(x)$ is not defined since $f$ oscillates quickly as $x to 0$.
$endgroup$
add a comment |
$begingroup$
Here's an example that is rather continuous.
Take $f(x) = sinleft(2pifrac{ln |x|}{ln 2}right)$ defined for $x neq 0$ (you can set $f(0)=0$ if you want $f$ to be definied for all $x in mathbb R$). Then
$$f(2x) = sinleft(2pifrac{ln |2x|}{ln 2}right) = sinleft(2pifrac{ln2 + ln |x|}{ln 2}right) \= sinleft(2pi+2pifrac{ln |x|}{ln 2}right) = sinleft(2pifrac{ln |x|}{ln 2}right) \= f(x)$$
so
$$lim_{xto 0}(f(x)-f(2x)) = 0.$$
But $lim_{xto 0} f(x)$ is not defined since $f$ oscillates quickly as $x to 0$.
$endgroup$
add a comment |
$begingroup$
Here's an example that is rather continuous.
Take $f(x) = sinleft(2pifrac{ln |x|}{ln 2}right)$ defined for $x neq 0$ (you can set $f(0)=0$ if you want $f$ to be definied for all $x in mathbb R$). Then
$$f(2x) = sinleft(2pifrac{ln |2x|}{ln 2}right) = sinleft(2pifrac{ln2 + ln |x|}{ln 2}right) \= sinleft(2pi+2pifrac{ln |x|}{ln 2}right) = sinleft(2pifrac{ln |x|}{ln 2}right) \= f(x)$$
so
$$lim_{xto 0}(f(x)-f(2x)) = 0.$$
But $lim_{xto 0} f(x)$ is not defined since $f$ oscillates quickly as $x to 0$.
$endgroup$
Here's an example that is rather continuous.
Take $f(x) = sinleft(2pifrac{ln |x|}{ln 2}right)$ defined for $x neq 0$ (you can set $f(0)=0$ if you want $f$ to be definied for all $x in mathbb R$). Then
$$f(2x) = sinleft(2pifrac{ln |2x|}{ln 2}right) = sinleft(2pifrac{ln2 + ln |x|}{ln 2}right) \= sinleft(2pi+2pifrac{ln |x|}{ln 2}right) = sinleft(2pifrac{ln |x|}{ln 2}right) \= f(x)$$
so
$$lim_{xto 0}(f(x)-f(2x)) = 0.$$
But $lim_{xto 0} f(x)$ is not defined since $f$ oscillates quickly as $x to 0$.
answered Dec 25 '17 at 9:25
md2perpemd2perpe
8,30911028
8,30911028
add a comment |
add a comment |
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$begingroup$
I guess $f(x)=sgn(x)$ would do the job. That is $f$ equals $1$, $0$ or $-1$ when $x$ is positive, null or negative, respectively.
$endgroup$
– Alejandro Nasif Salum
Dec 25 '17 at 8:38
$begingroup$
Does $pminfty$ count as existence of the limit?
$endgroup$
– Shashi
Dec 25 '17 at 8:44