Showing $f(z)=|z|^{1/2}z$ is differentiable at $z=0$ but not holomorphic.
$begingroup$
Given $f(z)=|z|^{1/2}z$, it is obviously complex differentiable at $z=0$ because $$f'(0)=lim_{zrightarrow 0}frac{|z|^{1/2}z}{z}=0$$ I have done similar examples with $|z|^2$, but the $|z|^{1/2}$ is throwing me off because you cannot simply expand it like you would with $|z|^2$. How can I show that it is not holomorphic at $z=0$?
Also, what would be the set of all $z$ such that $f$ is complex differentiable? I assume I have to use Cauchy-Riemann equation, but again, I'm having trouble working with the $|z|^{1/2}$
complex-analysis
$endgroup$
add a comment |
$begingroup$
Given $f(z)=|z|^{1/2}z$, it is obviously complex differentiable at $z=0$ because $$f'(0)=lim_{zrightarrow 0}frac{|z|^{1/2}z}{z}=0$$ I have done similar examples with $|z|^2$, but the $|z|^{1/2}$ is throwing me off because you cannot simply expand it like you would with $|z|^2$. How can I show that it is not holomorphic at $z=0$?
Also, what would be the set of all $z$ such that $f$ is complex differentiable? I assume I have to use Cauchy-Riemann equation, but again, I'm having trouble working with the $|z|^{1/2}$
complex-analysis
$endgroup$
$begingroup$
Have you tried looking at the partial derivatives and determining where they obey the Cauchy Riemann equations?
$endgroup$
– SmileyCraft
Dec 19 '18 at 15:33
$begingroup$
One approach might be (not sure if it works): since $g(z)=z^2$ is holomorphic, consider $gcirc f(z)$. I believe this fails to be holomorphic, so $f(z)$ is not holomorphic. I’m not $100%$ sure this is valid (though I am reasonably sure), but it at least provides a nice heuristic.
$endgroup$
– Clayton
Dec 19 '18 at 15:36
1
$begingroup$
@Kevin What for? And surely you are aware there exists no continuous square root in a neighborhood of the origin?
$endgroup$
– Did
Dec 19 '18 at 19:26
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@Did Mind blank. Yes I was aware, I had a bad day :-(
$endgroup$
– Kevin
Dec 20 '18 at 10:24
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@Kevin No problem. Next day surely will be better... :-)
$endgroup$
– Did
Dec 20 '18 at 11:29
add a comment |
$begingroup$
Given $f(z)=|z|^{1/2}z$, it is obviously complex differentiable at $z=0$ because $$f'(0)=lim_{zrightarrow 0}frac{|z|^{1/2}z}{z}=0$$ I have done similar examples with $|z|^2$, but the $|z|^{1/2}$ is throwing me off because you cannot simply expand it like you would with $|z|^2$. How can I show that it is not holomorphic at $z=0$?
Also, what would be the set of all $z$ such that $f$ is complex differentiable? I assume I have to use Cauchy-Riemann equation, but again, I'm having trouble working with the $|z|^{1/2}$
complex-analysis
$endgroup$
Given $f(z)=|z|^{1/2}z$, it is obviously complex differentiable at $z=0$ because $$f'(0)=lim_{zrightarrow 0}frac{|z|^{1/2}z}{z}=0$$ I have done similar examples with $|z|^2$, but the $|z|^{1/2}$ is throwing me off because you cannot simply expand it like you would with $|z|^2$. How can I show that it is not holomorphic at $z=0$?
Also, what would be the set of all $z$ such that $f$ is complex differentiable? I assume I have to use Cauchy-Riemann equation, but again, I'm having trouble working with the $|z|^{1/2}$
complex-analysis
complex-analysis
asked Dec 19 '18 at 15:25
Ya GYa G
519211
519211
$begingroup$
Have you tried looking at the partial derivatives and determining where they obey the Cauchy Riemann equations?
$endgroup$
– SmileyCraft
Dec 19 '18 at 15:33
$begingroup$
One approach might be (not sure if it works): since $g(z)=z^2$ is holomorphic, consider $gcirc f(z)$. I believe this fails to be holomorphic, so $f(z)$ is not holomorphic. I’m not $100%$ sure this is valid (though I am reasonably sure), but it at least provides a nice heuristic.
$endgroup$
– Clayton
Dec 19 '18 at 15:36
1
$begingroup$
@Kevin What for? And surely you are aware there exists no continuous square root in a neighborhood of the origin?
$endgroup$
– Did
Dec 19 '18 at 19:26
$begingroup$
@Did Mind blank. Yes I was aware, I had a bad day :-(
$endgroup$
– Kevin
Dec 20 '18 at 10:24
$begingroup$
@Kevin No problem. Next day surely will be better... :-)
$endgroup$
– Did
Dec 20 '18 at 11:29
add a comment |
$begingroup$
Have you tried looking at the partial derivatives and determining where they obey the Cauchy Riemann equations?
$endgroup$
– SmileyCraft
Dec 19 '18 at 15:33
$begingroup$
One approach might be (not sure if it works): since $g(z)=z^2$ is holomorphic, consider $gcirc f(z)$. I believe this fails to be holomorphic, so $f(z)$ is not holomorphic. I’m not $100%$ sure this is valid (though I am reasonably sure), but it at least provides a nice heuristic.
$endgroup$
– Clayton
Dec 19 '18 at 15:36
1
$begingroup$
@Kevin What for? And surely you are aware there exists no continuous square root in a neighborhood of the origin?
$endgroup$
– Did
Dec 19 '18 at 19:26
$begingroup$
@Did Mind blank. Yes I was aware, I had a bad day :-(
$endgroup$
– Kevin
Dec 20 '18 at 10:24
$begingroup$
@Kevin No problem. Next day surely will be better... :-)
$endgroup$
– Did
Dec 20 '18 at 11:29
$begingroup$
Have you tried looking at the partial derivatives and determining where they obey the Cauchy Riemann equations?
$endgroup$
– SmileyCraft
Dec 19 '18 at 15:33
$begingroup$
Have you tried looking at the partial derivatives and determining where they obey the Cauchy Riemann equations?
$endgroup$
– SmileyCraft
Dec 19 '18 at 15:33
$begingroup$
One approach might be (not sure if it works): since $g(z)=z^2$ is holomorphic, consider $gcirc f(z)$. I believe this fails to be holomorphic, so $f(z)$ is not holomorphic. I’m not $100%$ sure this is valid (though I am reasonably sure), but it at least provides a nice heuristic.
$endgroup$
– Clayton
Dec 19 '18 at 15:36
$begingroup$
One approach might be (not sure if it works): since $g(z)=z^2$ is holomorphic, consider $gcirc f(z)$. I believe this fails to be holomorphic, so $f(z)$ is not holomorphic. I’m not $100%$ sure this is valid (though I am reasonably sure), but it at least provides a nice heuristic.
$endgroup$
– Clayton
Dec 19 '18 at 15:36
1
1
$begingroup$
@Kevin What for? And surely you are aware there exists no continuous square root in a neighborhood of the origin?
$endgroup$
– Did
Dec 19 '18 at 19:26
$begingroup$
@Kevin What for? And surely you are aware there exists no continuous square root in a neighborhood of the origin?
$endgroup$
– Did
Dec 19 '18 at 19:26
$begingroup$
@Did Mind blank. Yes I was aware, I had a bad day :-(
$endgroup$
– Kevin
Dec 20 '18 at 10:24
$begingroup$
@Did Mind blank. Yes I was aware, I had a bad day :-(
$endgroup$
– Kevin
Dec 20 '18 at 10:24
$begingroup$
@Kevin No problem. Next day surely will be better... :-)
$endgroup$
– Did
Dec 20 '18 at 11:29
$begingroup$
@Kevin No problem. Next day surely will be better... :-)
$endgroup$
– Did
Dec 20 '18 at 11:29
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Holomorphicity at $0$ means that it's complex-differentiable in a neighbourhood
of $0$. But it's not complex-differentiable other than at $0$.
For the C-R equations, one has $f(x+iy)=u+iv$
where
$$u=x(x^2+y^2)^{1/4}$$
and
$$v=y(x^2+y^2)^{1/4}.$$
Then
$$u_x=(x^2+y^2)^{1/4}+frac{x^2}{2(x^2+y^2)^{3/4}}
=frac{3x^2+2y^2}{2(x^2+y^2)^{3/4}}$$
and similarly,
$$v_y=frac{2x^2+3y^2}{2(x^2+y^2)^{3/4}}.$$
These are different at nonzero points on the $x$-axis, so $f$ isn't
complex-differentiable there, and so not in any neighbourhood of $0$.
$endgroup$
$begingroup$
shouldn't $u=xleft(x^2+y^2right)^{1/4}$? since $|z|$ itself is $sqrt{x^2+y^2}$
$endgroup$
– Ya G
Dec 19 '18 at 15:37
$begingroup$
@YaG Thanks${}$!
$endgroup$
– Lord Shark the Unknown
Dec 19 '18 at 15:42
add a comment |
$begingroup$
Substistute $z = x+iy$
Separate your function into real parts and imaginary parts.
$|z|^frac12$ is strictly real
$|z| = (x^2 + y^2)^frac 12\
|z|^{frac 12} = (x^2 + y^2)^frac 14\
|z|^{frac 12}z = (x^2 + y^2)^frac 14(x+iy)$
$f(x+iy) = u+iv\
u = (x^2+y^2)^frac 14x\
v = (x^2+y^2)^frac 14y$
Do these satisfy they Cauchy-Reimann equations?
$frac {partial u}{partial x} = frac {partial v}{partial y}\
frac {partial u}{partial y} = -frac {partial v}{partial x}$
$endgroup$
$begingroup$
Got it! However, your $u$ and $v$ looks wrong.
$endgroup$
– Ya G
Dec 19 '18 at 15:59
add a comment |
Your Answer
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2 Answers
2
active
oldest
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2 Answers
2
active
oldest
votes
active
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votes
active
oldest
votes
$begingroup$
Holomorphicity at $0$ means that it's complex-differentiable in a neighbourhood
of $0$. But it's not complex-differentiable other than at $0$.
For the C-R equations, one has $f(x+iy)=u+iv$
where
$$u=x(x^2+y^2)^{1/4}$$
and
$$v=y(x^2+y^2)^{1/4}.$$
Then
$$u_x=(x^2+y^2)^{1/4}+frac{x^2}{2(x^2+y^2)^{3/4}}
=frac{3x^2+2y^2}{2(x^2+y^2)^{3/4}}$$
and similarly,
$$v_y=frac{2x^2+3y^2}{2(x^2+y^2)^{3/4}}.$$
These are different at nonzero points on the $x$-axis, so $f$ isn't
complex-differentiable there, and so not in any neighbourhood of $0$.
$endgroup$
$begingroup$
shouldn't $u=xleft(x^2+y^2right)^{1/4}$? since $|z|$ itself is $sqrt{x^2+y^2}$
$endgroup$
– Ya G
Dec 19 '18 at 15:37
$begingroup$
@YaG Thanks${}$!
$endgroup$
– Lord Shark the Unknown
Dec 19 '18 at 15:42
add a comment |
$begingroup$
Holomorphicity at $0$ means that it's complex-differentiable in a neighbourhood
of $0$. But it's not complex-differentiable other than at $0$.
For the C-R equations, one has $f(x+iy)=u+iv$
where
$$u=x(x^2+y^2)^{1/4}$$
and
$$v=y(x^2+y^2)^{1/4}.$$
Then
$$u_x=(x^2+y^2)^{1/4}+frac{x^2}{2(x^2+y^2)^{3/4}}
=frac{3x^2+2y^2}{2(x^2+y^2)^{3/4}}$$
and similarly,
$$v_y=frac{2x^2+3y^2}{2(x^2+y^2)^{3/4}}.$$
These are different at nonzero points on the $x$-axis, so $f$ isn't
complex-differentiable there, and so not in any neighbourhood of $0$.
$endgroup$
$begingroup$
shouldn't $u=xleft(x^2+y^2right)^{1/4}$? since $|z|$ itself is $sqrt{x^2+y^2}$
$endgroup$
– Ya G
Dec 19 '18 at 15:37
$begingroup$
@YaG Thanks${}$!
$endgroup$
– Lord Shark the Unknown
Dec 19 '18 at 15:42
add a comment |
$begingroup$
Holomorphicity at $0$ means that it's complex-differentiable in a neighbourhood
of $0$. But it's not complex-differentiable other than at $0$.
For the C-R equations, one has $f(x+iy)=u+iv$
where
$$u=x(x^2+y^2)^{1/4}$$
and
$$v=y(x^2+y^2)^{1/4}.$$
Then
$$u_x=(x^2+y^2)^{1/4}+frac{x^2}{2(x^2+y^2)^{3/4}}
=frac{3x^2+2y^2}{2(x^2+y^2)^{3/4}}$$
and similarly,
$$v_y=frac{2x^2+3y^2}{2(x^2+y^2)^{3/4}}.$$
These are different at nonzero points on the $x$-axis, so $f$ isn't
complex-differentiable there, and so not in any neighbourhood of $0$.
$endgroup$
Holomorphicity at $0$ means that it's complex-differentiable in a neighbourhood
of $0$. But it's not complex-differentiable other than at $0$.
For the C-R equations, one has $f(x+iy)=u+iv$
where
$$u=x(x^2+y^2)^{1/4}$$
and
$$v=y(x^2+y^2)^{1/4}.$$
Then
$$u_x=(x^2+y^2)^{1/4}+frac{x^2}{2(x^2+y^2)^{3/4}}
=frac{3x^2+2y^2}{2(x^2+y^2)^{3/4}}$$
and similarly,
$$v_y=frac{2x^2+3y^2}{2(x^2+y^2)^{3/4}}.$$
These are different at nonzero points on the $x$-axis, so $f$ isn't
complex-differentiable there, and so not in any neighbourhood of $0$.
edited Dec 19 '18 at 15:41
answered Dec 19 '18 at 15:34
Lord Shark the UnknownLord Shark the Unknown
105k1160132
105k1160132
$begingroup$
shouldn't $u=xleft(x^2+y^2right)^{1/4}$? since $|z|$ itself is $sqrt{x^2+y^2}$
$endgroup$
– Ya G
Dec 19 '18 at 15:37
$begingroup$
@YaG Thanks${}$!
$endgroup$
– Lord Shark the Unknown
Dec 19 '18 at 15:42
add a comment |
$begingroup$
shouldn't $u=xleft(x^2+y^2right)^{1/4}$? since $|z|$ itself is $sqrt{x^2+y^2}$
$endgroup$
– Ya G
Dec 19 '18 at 15:37
$begingroup$
@YaG Thanks${}$!
$endgroup$
– Lord Shark the Unknown
Dec 19 '18 at 15:42
$begingroup$
shouldn't $u=xleft(x^2+y^2right)^{1/4}$? since $|z|$ itself is $sqrt{x^2+y^2}$
$endgroup$
– Ya G
Dec 19 '18 at 15:37
$begingroup$
shouldn't $u=xleft(x^2+y^2right)^{1/4}$? since $|z|$ itself is $sqrt{x^2+y^2}$
$endgroup$
– Ya G
Dec 19 '18 at 15:37
$begingroup$
@YaG Thanks${}$!
$endgroup$
– Lord Shark the Unknown
Dec 19 '18 at 15:42
$begingroup$
@YaG Thanks${}$!
$endgroup$
– Lord Shark the Unknown
Dec 19 '18 at 15:42
add a comment |
$begingroup$
Substistute $z = x+iy$
Separate your function into real parts and imaginary parts.
$|z|^frac12$ is strictly real
$|z| = (x^2 + y^2)^frac 12\
|z|^{frac 12} = (x^2 + y^2)^frac 14\
|z|^{frac 12}z = (x^2 + y^2)^frac 14(x+iy)$
$f(x+iy) = u+iv\
u = (x^2+y^2)^frac 14x\
v = (x^2+y^2)^frac 14y$
Do these satisfy they Cauchy-Reimann equations?
$frac {partial u}{partial x} = frac {partial v}{partial y}\
frac {partial u}{partial y} = -frac {partial v}{partial x}$
$endgroup$
$begingroup$
Got it! However, your $u$ and $v$ looks wrong.
$endgroup$
– Ya G
Dec 19 '18 at 15:59
add a comment |
$begingroup$
Substistute $z = x+iy$
Separate your function into real parts and imaginary parts.
$|z|^frac12$ is strictly real
$|z| = (x^2 + y^2)^frac 12\
|z|^{frac 12} = (x^2 + y^2)^frac 14\
|z|^{frac 12}z = (x^2 + y^2)^frac 14(x+iy)$
$f(x+iy) = u+iv\
u = (x^2+y^2)^frac 14x\
v = (x^2+y^2)^frac 14y$
Do these satisfy they Cauchy-Reimann equations?
$frac {partial u}{partial x} = frac {partial v}{partial y}\
frac {partial u}{partial y} = -frac {partial v}{partial x}$
$endgroup$
$begingroup$
Got it! However, your $u$ and $v$ looks wrong.
$endgroup$
– Ya G
Dec 19 '18 at 15:59
add a comment |
$begingroup$
Substistute $z = x+iy$
Separate your function into real parts and imaginary parts.
$|z|^frac12$ is strictly real
$|z| = (x^2 + y^2)^frac 12\
|z|^{frac 12} = (x^2 + y^2)^frac 14\
|z|^{frac 12}z = (x^2 + y^2)^frac 14(x+iy)$
$f(x+iy) = u+iv\
u = (x^2+y^2)^frac 14x\
v = (x^2+y^2)^frac 14y$
Do these satisfy they Cauchy-Reimann equations?
$frac {partial u}{partial x} = frac {partial v}{partial y}\
frac {partial u}{partial y} = -frac {partial v}{partial x}$
$endgroup$
Substistute $z = x+iy$
Separate your function into real parts and imaginary parts.
$|z|^frac12$ is strictly real
$|z| = (x^2 + y^2)^frac 12\
|z|^{frac 12} = (x^2 + y^2)^frac 14\
|z|^{frac 12}z = (x^2 + y^2)^frac 14(x+iy)$
$f(x+iy) = u+iv\
u = (x^2+y^2)^frac 14x\
v = (x^2+y^2)^frac 14y$
Do these satisfy they Cauchy-Reimann equations?
$frac {partial u}{partial x} = frac {partial v}{partial y}\
frac {partial u}{partial y} = -frac {partial v}{partial x}$
edited Dec 19 '18 at 17:23
answered Dec 19 '18 at 15:57
Doug MDoug M
45.2k31954
45.2k31954
$begingroup$
Got it! However, your $u$ and $v$ looks wrong.
$endgroup$
– Ya G
Dec 19 '18 at 15:59
add a comment |
$begingroup$
Got it! However, your $u$ and $v$ looks wrong.
$endgroup$
– Ya G
Dec 19 '18 at 15:59
$begingroup$
Got it! However, your $u$ and $v$ looks wrong.
$endgroup$
– Ya G
Dec 19 '18 at 15:59
$begingroup$
Got it! However, your $u$ and $v$ looks wrong.
$endgroup$
– Ya G
Dec 19 '18 at 15:59
add a comment |
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$begingroup$
Have you tried looking at the partial derivatives and determining where they obey the Cauchy Riemann equations?
$endgroup$
– SmileyCraft
Dec 19 '18 at 15:33
$begingroup$
One approach might be (not sure if it works): since $g(z)=z^2$ is holomorphic, consider $gcirc f(z)$. I believe this fails to be holomorphic, so $f(z)$ is not holomorphic. I’m not $100%$ sure this is valid (though I am reasonably sure), but it at least provides a nice heuristic.
$endgroup$
– Clayton
Dec 19 '18 at 15:36
1
$begingroup$
@Kevin What for? And surely you are aware there exists no continuous square root in a neighborhood of the origin?
$endgroup$
– Did
Dec 19 '18 at 19:26
$begingroup$
@Did Mind blank. Yes I was aware, I had a bad day :-(
$endgroup$
– Kevin
Dec 20 '18 at 10:24
$begingroup$
@Kevin No problem. Next day surely will be better... :-)
$endgroup$
– Did
Dec 20 '18 at 11:29