If $f_nrightarrow f$ uniformly on compact subsets $Omega$ and $f$ is not constant, prove $f(Omega)subseteq...
$begingroup$
Let $Omega$ be open and connected, and let ${f_n}$ be a sequence of holomorphic functions on $Omega$ such that $f_n(Omega)subseteq Omega$. If $f_nrightarrow f$ uniformly on compact subsets $Omega$ and $f$ is not constant, prove $f(Omega)subseteq Omega$
My attempt:
Let $z_0in Omega$ and let $g(z)=f(z)-f(z_0)$ and $h(z)=f_n(z)-f(z_0)$. Clearly, $g(z)$ has a zero in a small neighborhood of $z_0$, which is $z_0$.
If I'm able to show that $h(z)$ also has a zero in that neighborhood, say $z_1$, this implies that $f(z_0)=f_n(z_1)in Omega$ and this completes the proof.
But I'm having trouble showing that $h(z)$ has a zero in the neighborhood of $z_0$. I'm trying to use Rouche's theorem by showing if $|g(z)-h(z)|<|g(z)|$ in the boundary of some neighborhood of $z_0$, $g(z)$ and $h(z)$ has the same number of roots in the neighborhood, hence, there exists some $z_1$ that satisfies my claim.
Can anyone show me how to get the inequality?
complex-analysis
$endgroup$
add a comment |
$begingroup$
Let $Omega$ be open and connected, and let ${f_n}$ be a sequence of holomorphic functions on $Omega$ such that $f_n(Omega)subseteq Omega$. If $f_nrightarrow f$ uniformly on compact subsets $Omega$ and $f$ is not constant, prove $f(Omega)subseteq Omega$
My attempt:
Let $z_0in Omega$ and let $g(z)=f(z)-f(z_0)$ and $h(z)=f_n(z)-f(z_0)$. Clearly, $g(z)$ has a zero in a small neighborhood of $z_0$, which is $z_0$.
If I'm able to show that $h(z)$ also has a zero in that neighborhood, say $z_1$, this implies that $f(z_0)=f_n(z_1)in Omega$ and this completes the proof.
But I'm having trouble showing that $h(z)$ has a zero in the neighborhood of $z_0$. I'm trying to use Rouche's theorem by showing if $|g(z)-h(z)|<|g(z)|$ in the boundary of some neighborhood of $z_0$, $g(z)$ and $h(z)$ has the same number of roots in the neighborhood, hence, there exists some $z_1$ that satisfies my claim.
Can anyone show me how to get the inequality?
complex-analysis
$endgroup$
add a comment |
$begingroup$
Let $Omega$ be open and connected, and let ${f_n}$ be a sequence of holomorphic functions on $Omega$ such that $f_n(Omega)subseteq Omega$. If $f_nrightarrow f$ uniformly on compact subsets $Omega$ and $f$ is not constant, prove $f(Omega)subseteq Omega$
My attempt:
Let $z_0in Omega$ and let $g(z)=f(z)-f(z_0)$ and $h(z)=f_n(z)-f(z_0)$. Clearly, $g(z)$ has a zero in a small neighborhood of $z_0$, which is $z_0$.
If I'm able to show that $h(z)$ also has a zero in that neighborhood, say $z_1$, this implies that $f(z_0)=f_n(z_1)in Omega$ and this completes the proof.
But I'm having trouble showing that $h(z)$ has a zero in the neighborhood of $z_0$. I'm trying to use Rouche's theorem by showing if $|g(z)-h(z)|<|g(z)|$ in the boundary of some neighborhood of $z_0$, $g(z)$ and $h(z)$ has the same number of roots in the neighborhood, hence, there exists some $z_1$ that satisfies my claim.
Can anyone show me how to get the inequality?
complex-analysis
$endgroup$
Let $Omega$ be open and connected, and let ${f_n}$ be a sequence of holomorphic functions on $Omega$ such that $f_n(Omega)subseteq Omega$. If $f_nrightarrow f$ uniformly on compact subsets $Omega$ and $f$ is not constant, prove $f(Omega)subseteq Omega$
My attempt:
Let $z_0in Omega$ and let $g(z)=f(z)-f(z_0)$ and $h(z)=f_n(z)-f(z_0)$. Clearly, $g(z)$ has a zero in a small neighborhood of $z_0$, which is $z_0$.
If I'm able to show that $h(z)$ also has a zero in that neighborhood, say $z_1$, this implies that $f(z_0)=f_n(z_1)in Omega$ and this completes the proof.
But I'm having trouble showing that $h(z)$ has a zero in the neighborhood of $z_0$. I'm trying to use Rouche's theorem by showing if $|g(z)-h(z)|<|g(z)|$ in the boundary of some neighborhood of $z_0$, $g(z)$ and $h(z)$ has the same number of roots in the neighborhood, hence, there exists some $z_1$ that satisfies my claim.
Can anyone show me how to get the inequality?
complex-analysis
complex-analysis
asked Dec 19 '18 at 18:44
Ya GYa G
529211
529211
add a comment |
add a comment |
1 Answer
1
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Suppose for the sake of contradiction there exists a point $z_0$ such that $z_0 notin Omega$, such that $f(w_0) = z_0$. WLOG $z_0 = 0$, then by the argument principle begin{equation} int_{gamma}
frac{f'}{f} geq 1 end{equation} where $gamma$ is a small contour around zero so that it doesn't intersect $Omega$; this is possible since $Omega$ is open. But observe begin{equation} int_{gamma} frac{f_n'}{f_n} = 0 end{equation} for any $n$ since $f_n(Omega) subset Omega$, so $f_n neq 0$ for any $z in Omega$. So by uniform convergence we get begin{equation} int_{gamma}
frac{f'}{f} = 0 end{equation} which is our contradiction. In general uniform convergence + argument principle gives you many strong results such as if $f_n$ is $1-1$ and converges uniformly to $f$, then either $f$ is constant or $1-1$ and similarly if $f_n neq 0$ for any $z$. (Uniform convergence allows you to pass limit to integrals, and this integral tells you how many zeros there are.)
$endgroup$
$begingroup$
So the argument principle states that if $f$ is meromorphic with roots and poles not on the contour $gamma$, we have $frac{1}{2pi i}int_{gamma}frac{f'}{f}=$ number of zeros in $gamma$ - number of poles in $gamma$. How do we know that $int_{gamma}frac{f'}{f}geq1$? I guess I just need some more explanation on your reasoning.
$endgroup$
– Ya G
Dec 19 '18 at 19:22
$begingroup$
As $f_n rightarrow f$ uniformly on compact sets, $f_n$ holomorphic, we know by Morerra Theorem and uniform convergence that $f$ is holomorphic. So it has no poles, but $f(z_0) = 0$, so it has at least one zero in $gamma$ namely $z_0$. So the integral is greater than or equal to 1.
$endgroup$
– Story123
Dec 19 '18 at 19:26
1
$begingroup$
Got it! So you are assuming that there is a root inside the contour such that the root itself is not in $Omega$. But since it's inside the contour, the argument principle suggests that there exists at least one zero in the contour.
$endgroup$
– Ya G
Dec 19 '18 at 19:31
$begingroup$
Yes, you can assume WLOG it's a root since if it isn't consider $g(z) := f(z) - w_0$, then $g'(z) = f'(z)$ and you can apply the argument principle to $g$.
$endgroup$
– Story123
Dec 19 '18 at 19:36
$begingroup$
What if $z_0in partial Omega?$ Then every nbhd of $z_0$ intersects $Omega$
$endgroup$
– Matematleta
Dec 19 '18 at 22:06
add a comment |
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$begingroup$
Suppose for the sake of contradiction there exists a point $z_0$ such that $z_0 notin Omega$, such that $f(w_0) = z_0$. WLOG $z_0 = 0$, then by the argument principle begin{equation} int_{gamma}
frac{f'}{f} geq 1 end{equation} where $gamma$ is a small contour around zero so that it doesn't intersect $Omega$; this is possible since $Omega$ is open. But observe begin{equation} int_{gamma} frac{f_n'}{f_n} = 0 end{equation} for any $n$ since $f_n(Omega) subset Omega$, so $f_n neq 0$ for any $z in Omega$. So by uniform convergence we get begin{equation} int_{gamma}
frac{f'}{f} = 0 end{equation} which is our contradiction. In general uniform convergence + argument principle gives you many strong results such as if $f_n$ is $1-1$ and converges uniformly to $f$, then either $f$ is constant or $1-1$ and similarly if $f_n neq 0$ for any $z$. (Uniform convergence allows you to pass limit to integrals, and this integral tells you how many zeros there are.)
$endgroup$
$begingroup$
So the argument principle states that if $f$ is meromorphic with roots and poles not on the contour $gamma$, we have $frac{1}{2pi i}int_{gamma}frac{f'}{f}=$ number of zeros in $gamma$ - number of poles in $gamma$. How do we know that $int_{gamma}frac{f'}{f}geq1$? I guess I just need some more explanation on your reasoning.
$endgroup$
– Ya G
Dec 19 '18 at 19:22
$begingroup$
As $f_n rightarrow f$ uniformly on compact sets, $f_n$ holomorphic, we know by Morerra Theorem and uniform convergence that $f$ is holomorphic. So it has no poles, but $f(z_0) = 0$, so it has at least one zero in $gamma$ namely $z_0$. So the integral is greater than or equal to 1.
$endgroup$
– Story123
Dec 19 '18 at 19:26
1
$begingroup$
Got it! So you are assuming that there is a root inside the contour such that the root itself is not in $Omega$. But since it's inside the contour, the argument principle suggests that there exists at least one zero in the contour.
$endgroup$
– Ya G
Dec 19 '18 at 19:31
$begingroup$
Yes, you can assume WLOG it's a root since if it isn't consider $g(z) := f(z) - w_0$, then $g'(z) = f'(z)$ and you can apply the argument principle to $g$.
$endgroup$
– Story123
Dec 19 '18 at 19:36
$begingroup$
What if $z_0in partial Omega?$ Then every nbhd of $z_0$ intersects $Omega$
$endgroup$
– Matematleta
Dec 19 '18 at 22:06
add a comment |
$begingroup$
Suppose for the sake of contradiction there exists a point $z_0$ such that $z_0 notin Omega$, such that $f(w_0) = z_0$. WLOG $z_0 = 0$, then by the argument principle begin{equation} int_{gamma}
frac{f'}{f} geq 1 end{equation} where $gamma$ is a small contour around zero so that it doesn't intersect $Omega$; this is possible since $Omega$ is open. But observe begin{equation} int_{gamma} frac{f_n'}{f_n} = 0 end{equation} for any $n$ since $f_n(Omega) subset Omega$, so $f_n neq 0$ for any $z in Omega$. So by uniform convergence we get begin{equation} int_{gamma}
frac{f'}{f} = 0 end{equation} which is our contradiction. In general uniform convergence + argument principle gives you many strong results such as if $f_n$ is $1-1$ and converges uniformly to $f$, then either $f$ is constant or $1-1$ and similarly if $f_n neq 0$ for any $z$. (Uniform convergence allows you to pass limit to integrals, and this integral tells you how many zeros there are.)
$endgroup$
$begingroup$
So the argument principle states that if $f$ is meromorphic with roots and poles not on the contour $gamma$, we have $frac{1}{2pi i}int_{gamma}frac{f'}{f}=$ number of zeros in $gamma$ - number of poles in $gamma$. How do we know that $int_{gamma}frac{f'}{f}geq1$? I guess I just need some more explanation on your reasoning.
$endgroup$
– Ya G
Dec 19 '18 at 19:22
$begingroup$
As $f_n rightarrow f$ uniformly on compact sets, $f_n$ holomorphic, we know by Morerra Theorem and uniform convergence that $f$ is holomorphic. So it has no poles, but $f(z_0) = 0$, so it has at least one zero in $gamma$ namely $z_0$. So the integral is greater than or equal to 1.
$endgroup$
– Story123
Dec 19 '18 at 19:26
1
$begingroup$
Got it! So you are assuming that there is a root inside the contour such that the root itself is not in $Omega$. But since it's inside the contour, the argument principle suggests that there exists at least one zero in the contour.
$endgroup$
– Ya G
Dec 19 '18 at 19:31
$begingroup$
Yes, you can assume WLOG it's a root since if it isn't consider $g(z) := f(z) - w_0$, then $g'(z) = f'(z)$ and you can apply the argument principle to $g$.
$endgroup$
– Story123
Dec 19 '18 at 19:36
$begingroup$
What if $z_0in partial Omega?$ Then every nbhd of $z_0$ intersects $Omega$
$endgroup$
– Matematleta
Dec 19 '18 at 22:06
add a comment |
$begingroup$
Suppose for the sake of contradiction there exists a point $z_0$ such that $z_0 notin Omega$, such that $f(w_0) = z_0$. WLOG $z_0 = 0$, then by the argument principle begin{equation} int_{gamma}
frac{f'}{f} geq 1 end{equation} where $gamma$ is a small contour around zero so that it doesn't intersect $Omega$; this is possible since $Omega$ is open. But observe begin{equation} int_{gamma} frac{f_n'}{f_n} = 0 end{equation} for any $n$ since $f_n(Omega) subset Omega$, so $f_n neq 0$ for any $z in Omega$. So by uniform convergence we get begin{equation} int_{gamma}
frac{f'}{f} = 0 end{equation} which is our contradiction. In general uniform convergence + argument principle gives you many strong results such as if $f_n$ is $1-1$ and converges uniformly to $f$, then either $f$ is constant or $1-1$ and similarly if $f_n neq 0$ for any $z$. (Uniform convergence allows you to pass limit to integrals, and this integral tells you how many zeros there are.)
$endgroup$
Suppose for the sake of contradiction there exists a point $z_0$ such that $z_0 notin Omega$, such that $f(w_0) = z_0$. WLOG $z_0 = 0$, then by the argument principle begin{equation} int_{gamma}
frac{f'}{f} geq 1 end{equation} where $gamma$ is a small contour around zero so that it doesn't intersect $Omega$; this is possible since $Omega$ is open. But observe begin{equation} int_{gamma} frac{f_n'}{f_n} = 0 end{equation} for any $n$ since $f_n(Omega) subset Omega$, so $f_n neq 0$ for any $z in Omega$. So by uniform convergence we get begin{equation} int_{gamma}
frac{f'}{f} = 0 end{equation} which is our contradiction. In general uniform convergence + argument principle gives you many strong results such as if $f_n$ is $1-1$ and converges uniformly to $f$, then either $f$ is constant or $1-1$ and similarly if $f_n neq 0$ for any $z$. (Uniform convergence allows you to pass limit to integrals, and this integral tells you how many zeros there are.)
edited Dec 19 '18 at 19:01
answered Dec 19 '18 at 18:55
Story123Story123
23718
23718
$begingroup$
So the argument principle states that if $f$ is meromorphic with roots and poles not on the contour $gamma$, we have $frac{1}{2pi i}int_{gamma}frac{f'}{f}=$ number of zeros in $gamma$ - number of poles in $gamma$. How do we know that $int_{gamma}frac{f'}{f}geq1$? I guess I just need some more explanation on your reasoning.
$endgroup$
– Ya G
Dec 19 '18 at 19:22
$begingroup$
As $f_n rightarrow f$ uniformly on compact sets, $f_n$ holomorphic, we know by Morerra Theorem and uniform convergence that $f$ is holomorphic. So it has no poles, but $f(z_0) = 0$, so it has at least one zero in $gamma$ namely $z_0$. So the integral is greater than or equal to 1.
$endgroup$
– Story123
Dec 19 '18 at 19:26
1
$begingroup$
Got it! So you are assuming that there is a root inside the contour such that the root itself is not in $Omega$. But since it's inside the contour, the argument principle suggests that there exists at least one zero in the contour.
$endgroup$
– Ya G
Dec 19 '18 at 19:31
$begingroup$
Yes, you can assume WLOG it's a root since if it isn't consider $g(z) := f(z) - w_0$, then $g'(z) = f'(z)$ and you can apply the argument principle to $g$.
$endgroup$
– Story123
Dec 19 '18 at 19:36
$begingroup$
What if $z_0in partial Omega?$ Then every nbhd of $z_0$ intersects $Omega$
$endgroup$
– Matematleta
Dec 19 '18 at 22:06
add a comment |
$begingroup$
So the argument principle states that if $f$ is meromorphic with roots and poles not on the contour $gamma$, we have $frac{1}{2pi i}int_{gamma}frac{f'}{f}=$ number of zeros in $gamma$ - number of poles in $gamma$. How do we know that $int_{gamma}frac{f'}{f}geq1$? I guess I just need some more explanation on your reasoning.
$endgroup$
– Ya G
Dec 19 '18 at 19:22
$begingroup$
As $f_n rightarrow f$ uniformly on compact sets, $f_n$ holomorphic, we know by Morerra Theorem and uniform convergence that $f$ is holomorphic. So it has no poles, but $f(z_0) = 0$, so it has at least one zero in $gamma$ namely $z_0$. So the integral is greater than or equal to 1.
$endgroup$
– Story123
Dec 19 '18 at 19:26
1
$begingroup$
Got it! So you are assuming that there is a root inside the contour such that the root itself is not in $Omega$. But since it's inside the contour, the argument principle suggests that there exists at least one zero in the contour.
$endgroup$
– Ya G
Dec 19 '18 at 19:31
$begingroup$
Yes, you can assume WLOG it's a root since if it isn't consider $g(z) := f(z) - w_0$, then $g'(z) = f'(z)$ and you can apply the argument principle to $g$.
$endgroup$
– Story123
Dec 19 '18 at 19:36
$begingroup$
What if $z_0in partial Omega?$ Then every nbhd of $z_0$ intersects $Omega$
$endgroup$
– Matematleta
Dec 19 '18 at 22:06
$begingroup$
So the argument principle states that if $f$ is meromorphic with roots and poles not on the contour $gamma$, we have $frac{1}{2pi i}int_{gamma}frac{f'}{f}=$ number of zeros in $gamma$ - number of poles in $gamma$. How do we know that $int_{gamma}frac{f'}{f}geq1$? I guess I just need some more explanation on your reasoning.
$endgroup$
– Ya G
Dec 19 '18 at 19:22
$begingroup$
So the argument principle states that if $f$ is meromorphic with roots and poles not on the contour $gamma$, we have $frac{1}{2pi i}int_{gamma}frac{f'}{f}=$ number of zeros in $gamma$ - number of poles in $gamma$. How do we know that $int_{gamma}frac{f'}{f}geq1$? I guess I just need some more explanation on your reasoning.
$endgroup$
– Ya G
Dec 19 '18 at 19:22
$begingroup$
As $f_n rightarrow f$ uniformly on compact sets, $f_n$ holomorphic, we know by Morerra Theorem and uniform convergence that $f$ is holomorphic. So it has no poles, but $f(z_0) = 0$, so it has at least one zero in $gamma$ namely $z_0$. So the integral is greater than or equal to 1.
$endgroup$
– Story123
Dec 19 '18 at 19:26
$begingroup$
As $f_n rightarrow f$ uniformly on compact sets, $f_n$ holomorphic, we know by Morerra Theorem and uniform convergence that $f$ is holomorphic. So it has no poles, but $f(z_0) = 0$, so it has at least one zero in $gamma$ namely $z_0$. So the integral is greater than or equal to 1.
$endgroup$
– Story123
Dec 19 '18 at 19:26
1
1
$begingroup$
Got it! So you are assuming that there is a root inside the contour such that the root itself is not in $Omega$. But since it's inside the contour, the argument principle suggests that there exists at least one zero in the contour.
$endgroup$
– Ya G
Dec 19 '18 at 19:31
$begingroup$
Got it! So you are assuming that there is a root inside the contour such that the root itself is not in $Omega$. But since it's inside the contour, the argument principle suggests that there exists at least one zero in the contour.
$endgroup$
– Ya G
Dec 19 '18 at 19:31
$begingroup$
Yes, you can assume WLOG it's a root since if it isn't consider $g(z) := f(z) - w_0$, then $g'(z) = f'(z)$ and you can apply the argument principle to $g$.
$endgroup$
– Story123
Dec 19 '18 at 19:36
$begingroup$
Yes, you can assume WLOG it's a root since if it isn't consider $g(z) := f(z) - w_0$, then $g'(z) = f'(z)$ and you can apply the argument principle to $g$.
$endgroup$
– Story123
Dec 19 '18 at 19:36
$begingroup$
What if $z_0in partial Omega?$ Then every nbhd of $z_0$ intersects $Omega$
$endgroup$
– Matematleta
Dec 19 '18 at 22:06
$begingroup$
What if $z_0in partial Omega?$ Then every nbhd of $z_0$ intersects $Omega$
$endgroup$
– Matematleta
Dec 19 '18 at 22:06
add a comment |
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