Showing $f(z)=|z|^{1/2}z$ is differentiable at $z=0$ but not holomorphic.












2












$begingroup$


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 $|z|^{1/2}$










share|cite|improve this question









$endgroup$












  • $begingroup$
    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










  • $begingroup$
    @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
















2












$begingroup$


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 $|z|^{1/2}$










share|cite|improve this question









$endgroup$












  • $begingroup$
    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










  • $begingroup$
    @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














2












2








2


1



$begingroup$


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 $|z|^{1/2}$










share|cite|improve this question









$endgroup$




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






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










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


















  • $begingroup$
    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










2 Answers
2






active

oldest

votes


















4












$begingroup$

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 $0$.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    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



















0












$begingroup$

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?



$frac {partial u}{partial x} = frac {partial v}{partial y}\
frac {partial u}{partial y} = -frac {partial v}{partial x}$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Got it! However, your $u$ and $v$ looks wrong.
    $endgroup$
    – Ya G
    Dec 19 '18 at 15:59











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2 Answers
2






active

oldest

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2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









4












$begingroup$

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 $0$.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    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
















4












$begingroup$

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 $0$.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    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














4












4








4





$begingroup$

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 $0$.






share|cite|improve this answer











$endgroup$



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 $0$.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








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


















  • $begingroup$
    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











0












$begingroup$

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?



$frac {partial u}{partial x} = frac {partial v}{partial y}\
frac {partial u}{partial y} = -frac {partial v}{partial x}$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Got it! However, your $u$ and $v$ looks wrong.
    $endgroup$
    – Ya G
    Dec 19 '18 at 15:59
















0












$begingroup$

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?



$frac {partial u}{partial x} = frac {partial v}{partial y}\
frac {partial u}{partial y} = -frac {partial v}{partial x}$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Got it! However, your $u$ and $v$ looks wrong.
    $endgroup$
    – Ya G
    Dec 19 '18 at 15:59














0












0








0





$begingroup$

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?



$frac {partial u}{partial x} = frac {partial v}{partial y}\
frac {partial u}{partial y} = -frac {partial v}{partial x}$






share|cite|improve this answer











$endgroup$



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?



$frac {partial u}{partial x} = frac {partial v}{partial y}\
frac {partial u}{partial y} = -frac {partial v}{partial x}$







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








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


















  • $begingroup$
    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


















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