Proving theorem connecting the inverse of a holomorphic function to a contour integral of the function.
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I am asked to prove this theorem:
If $f:U rightarrow C$ is holomorphic in $U$ and invertible, $Pin U$ and if $D(P,r)$ is a sufficently small disc about P, then
$$f^{-1}(w) = frac{1}{2pi i} oint_{partial D(P,r)}{frac{sf'(s)}{f(s)-w}}ds$$
The book says to "imitate the proof of the argument principle" but I am not seeing the connection.
complex-analysis
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add a comment |
$begingroup$
I am asked to prove this theorem:
If $f:U rightarrow C$ is holomorphic in $U$ and invertible, $Pin U$ and if $D(P,r)$ is a sufficently small disc about P, then
$$f^{-1}(w) = frac{1}{2pi i} oint_{partial D(P,r)}{frac{sf'(s)}{f(s)-w}}ds$$
The book says to "imitate the proof of the argument principle" but I am not seeing the connection.
complex-analysis
$endgroup$
$begingroup$
You should assume that $|f^{-1}(w)-P|<r.$
$endgroup$
– P..
Nov 12 '12 at 7:06
add a comment |
$begingroup$
I am asked to prove this theorem:
If $f:U rightarrow C$ is holomorphic in $U$ and invertible, $Pin U$ and if $D(P,r)$ is a sufficently small disc about P, then
$$f^{-1}(w) = frac{1}{2pi i} oint_{partial D(P,r)}{frac{sf'(s)}{f(s)-w}}ds$$
The book says to "imitate the proof of the argument principle" but I am not seeing the connection.
complex-analysis
$endgroup$
I am asked to prove this theorem:
If $f:U rightarrow C$ is holomorphic in $U$ and invertible, $Pin U$ and if $D(P,r)$ is a sufficently small disc about P, then
$$f^{-1}(w) = frac{1}{2pi i} oint_{partial D(P,r)}{frac{sf'(s)}{f(s)-w}}ds$$
The book says to "imitate the proof of the argument principle" but I am not seeing the connection.
complex-analysis
complex-analysis
edited Nov 12 '12 at 5:56
Gerry Myerson
146k8147299
146k8147299
asked Nov 12 '12 at 5:33
MikeMike
166239
166239
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You should assume that $|f^{-1}(w)-P|<r.$
$endgroup$
– P..
Nov 12 '12 at 7:06
add a comment |
$begingroup$
You should assume that $|f^{-1}(w)-P|<r.$
$endgroup$
– P..
Nov 12 '12 at 7:06
$begingroup$
You should assume that $|f^{-1}(w)-P|<r.$
$endgroup$
– P..
Nov 12 '12 at 7:06
$begingroup$
You should assume that $|f^{-1}(w)-P|<r.$
$endgroup$
– P..
Nov 12 '12 at 7:06
add a comment |
3 Answers
3
active
oldest
votes
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Hint: Since $f$ is holomorphic and invertible, for each $win f(D(P,r))$, $f(z)-w$ has a unique zero $f^{-1}(w):=z_0$ in $U$; moreover, $z_0in D(P,r)$. Therefore, $f(z)=w+(z-z_0)h(z)$ on $U$, where $h$ is holomorphic and has no zero on $U$.
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add a comment |
$begingroup$
I had a similar problem:
- $f(z)$ has local inverse $f^{-1}(w) = z$. Write down an integral formula that gives $f^{-1}(w)$ in terms of $f(z)$.
My solution:
- Apply Cauchy Integral formula: Since $f^{-1}(w)$ is analytic by the inverse function theorem, we can say:
$$f^{-1}(w) = frac{1}{2pi i }oint_{f(partial D(P,r))}frac{f^{-1}(u)}{u-w}du$$
2. Using Substitution: $u=f(z)$ for $z$ on $partial D(P,r) implies du = f'(z)dz$
3.Rewrite: $$f^{-1}(w) = frac{1}{2pi i }oint_{partial D(P,r)}frac{zf'(z)}{f(z)-w}dz$$
$endgroup$
add a comment |
$begingroup$
After some thought, it makes sense after applying the Cauchy Integral Formula to the inverse function and then making the substitution $ f^{-1}(s) = t $ where t is taken along the path given by $f^{-1}(s)$ where s is along $partial D(P,r)$.
The problem is being able to deform that curve into a circle which goes around $f^{-1}(w)$. This can be done if we can show the path only goes around $f^{-1}(w)$ once.
I still have no idea how the argument principle is involved
$endgroup$
$begingroup$
It's just something very similar to the argument principle (in particular the proof), eg, see the answer. Or just look at the special case $w = 0$, and compare the statements and the proofs of this and argument principle.
$endgroup$
– user27126
Nov 15 '12 at 17:59
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I would but I still don't have a proof of this, can you direct me to one?
$endgroup$
– Mathstudent
Nov 15 '12 at 18:15
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What's wrong with the other answer?
$endgroup$
– user27126
Nov 15 '12 at 18:57
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The other answer is a hint, not a proof of the the above, which I am still confused on.
$endgroup$
– Mathstudent
Nov 19 '12 at 2:17
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OK. Use Cauchy's integral formula. There is only one pole to be picked up on the right hand side, which is at $z_0 = f^{-1}(w)$. So we only need to compute the residue. Near $z_0$, write $f(z)$ as $w + (z-z_0)^k h(z)$, where $h(z_0) neq 0$. Then $f'(z) = k(z-z_0)^{k-1} (h(z) + (z-z_0)h'(z)/k)$, where the thing inside the bracket does not vanish at $z_0$. Can you see what the residue at $z_0$ is now?
$endgroup$
– user27126
Nov 19 '12 at 6:53
add a comment |
Your Answer
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3 Answers
3
active
oldest
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3 Answers
3
active
oldest
votes
active
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$begingroup$
Hint: Since $f$ is holomorphic and invertible, for each $win f(D(P,r))$, $f(z)-w$ has a unique zero $f^{-1}(w):=z_0$ in $U$; moreover, $z_0in D(P,r)$. Therefore, $f(z)=w+(z-z_0)h(z)$ on $U$, where $h$ is holomorphic and has no zero on $U$.
$endgroup$
add a comment |
$begingroup$
Hint: Since $f$ is holomorphic and invertible, for each $win f(D(P,r))$, $f(z)-w$ has a unique zero $f^{-1}(w):=z_0$ in $U$; moreover, $z_0in D(P,r)$. Therefore, $f(z)=w+(z-z_0)h(z)$ on $U$, where $h$ is holomorphic and has no zero on $U$.
$endgroup$
add a comment |
$begingroup$
Hint: Since $f$ is holomorphic and invertible, for each $win f(D(P,r))$, $f(z)-w$ has a unique zero $f^{-1}(w):=z_0$ in $U$; moreover, $z_0in D(P,r)$. Therefore, $f(z)=w+(z-z_0)h(z)$ on $U$, where $h$ is holomorphic and has no zero on $U$.
$endgroup$
Hint: Since $f$ is holomorphic and invertible, for each $win f(D(P,r))$, $f(z)-w$ has a unique zero $f^{-1}(w):=z_0$ in $U$; moreover, $z_0in D(P,r)$. Therefore, $f(z)=w+(z-z_0)h(z)$ on $U$, where $h$ is holomorphic and has no zero on $U$.
answered Nov 12 '12 at 6:42
23rd23rd
13.9k2957
13.9k2957
add a comment |
add a comment |
$begingroup$
I had a similar problem:
- $f(z)$ has local inverse $f^{-1}(w) = z$. Write down an integral formula that gives $f^{-1}(w)$ in terms of $f(z)$.
My solution:
- Apply Cauchy Integral formula: Since $f^{-1}(w)$ is analytic by the inverse function theorem, we can say:
$$f^{-1}(w) = frac{1}{2pi i }oint_{f(partial D(P,r))}frac{f^{-1}(u)}{u-w}du$$
2. Using Substitution: $u=f(z)$ for $z$ on $partial D(P,r) implies du = f'(z)dz$
3.Rewrite: $$f^{-1}(w) = frac{1}{2pi i }oint_{partial D(P,r)}frac{zf'(z)}{f(z)-w}dz$$
$endgroup$
add a comment |
$begingroup$
I had a similar problem:
- $f(z)$ has local inverse $f^{-1}(w) = z$. Write down an integral formula that gives $f^{-1}(w)$ in terms of $f(z)$.
My solution:
- Apply Cauchy Integral formula: Since $f^{-1}(w)$ is analytic by the inverse function theorem, we can say:
$$f^{-1}(w) = frac{1}{2pi i }oint_{f(partial D(P,r))}frac{f^{-1}(u)}{u-w}du$$
2. Using Substitution: $u=f(z)$ for $z$ on $partial D(P,r) implies du = f'(z)dz$
3.Rewrite: $$f^{-1}(w) = frac{1}{2pi i }oint_{partial D(P,r)}frac{zf'(z)}{f(z)-w}dz$$
$endgroup$
add a comment |
$begingroup$
I had a similar problem:
- $f(z)$ has local inverse $f^{-1}(w) = z$. Write down an integral formula that gives $f^{-1}(w)$ in terms of $f(z)$.
My solution:
- Apply Cauchy Integral formula: Since $f^{-1}(w)$ is analytic by the inverse function theorem, we can say:
$$f^{-1}(w) = frac{1}{2pi i }oint_{f(partial D(P,r))}frac{f^{-1}(u)}{u-w}du$$
2. Using Substitution: $u=f(z)$ for $z$ on $partial D(P,r) implies du = f'(z)dz$
3.Rewrite: $$f^{-1}(w) = frac{1}{2pi i }oint_{partial D(P,r)}frac{zf'(z)}{f(z)-w}dz$$
$endgroup$
I had a similar problem:
- $f(z)$ has local inverse $f^{-1}(w) = z$. Write down an integral formula that gives $f^{-1}(w)$ in terms of $f(z)$.
My solution:
- Apply Cauchy Integral formula: Since $f^{-1}(w)$ is analytic by the inverse function theorem, we can say:
$$f^{-1}(w) = frac{1}{2pi i }oint_{f(partial D(P,r))}frac{f^{-1}(u)}{u-w}du$$
2. Using Substitution: $u=f(z)$ for $z$ on $partial D(P,r) implies du = f'(z)dz$
3.Rewrite: $$f^{-1}(w) = frac{1}{2pi i }oint_{partial D(P,r)}frac{zf'(z)}{f(z)-w}dz$$
answered Jul 25 '14 at 21:53
J_Lopez8J_Lopez8
555
555
add a comment |
add a comment |
$begingroup$
After some thought, it makes sense after applying the Cauchy Integral Formula to the inverse function and then making the substitution $ f^{-1}(s) = t $ where t is taken along the path given by $f^{-1}(s)$ where s is along $partial D(P,r)$.
The problem is being able to deform that curve into a circle which goes around $f^{-1}(w)$. This can be done if we can show the path only goes around $f^{-1}(w)$ once.
I still have no idea how the argument principle is involved
$endgroup$
$begingroup$
It's just something very similar to the argument principle (in particular the proof), eg, see the answer. Or just look at the special case $w = 0$, and compare the statements and the proofs of this and argument principle.
$endgroup$
– user27126
Nov 15 '12 at 17:59
$begingroup$
I would but I still don't have a proof of this, can you direct me to one?
$endgroup$
– Mathstudent
Nov 15 '12 at 18:15
$begingroup$
What's wrong with the other answer?
$endgroup$
– user27126
Nov 15 '12 at 18:57
$begingroup$
The other answer is a hint, not a proof of the the above, which I am still confused on.
$endgroup$
– Mathstudent
Nov 19 '12 at 2:17
$begingroup$
OK. Use Cauchy's integral formula. There is only one pole to be picked up on the right hand side, which is at $z_0 = f^{-1}(w)$. So we only need to compute the residue. Near $z_0$, write $f(z)$ as $w + (z-z_0)^k h(z)$, where $h(z_0) neq 0$. Then $f'(z) = k(z-z_0)^{k-1} (h(z) + (z-z_0)h'(z)/k)$, where the thing inside the bracket does not vanish at $z_0$. Can you see what the residue at $z_0$ is now?
$endgroup$
– user27126
Nov 19 '12 at 6:53
add a comment |
$begingroup$
After some thought, it makes sense after applying the Cauchy Integral Formula to the inverse function and then making the substitution $ f^{-1}(s) = t $ where t is taken along the path given by $f^{-1}(s)$ where s is along $partial D(P,r)$.
The problem is being able to deform that curve into a circle which goes around $f^{-1}(w)$. This can be done if we can show the path only goes around $f^{-1}(w)$ once.
I still have no idea how the argument principle is involved
$endgroup$
$begingroup$
It's just something very similar to the argument principle (in particular the proof), eg, see the answer. Or just look at the special case $w = 0$, and compare the statements and the proofs of this and argument principle.
$endgroup$
– user27126
Nov 15 '12 at 17:59
$begingroup$
I would but I still don't have a proof of this, can you direct me to one?
$endgroup$
– Mathstudent
Nov 15 '12 at 18:15
$begingroup$
What's wrong with the other answer?
$endgroup$
– user27126
Nov 15 '12 at 18:57
$begingroup$
The other answer is a hint, not a proof of the the above, which I am still confused on.
$endgroup$
– Mathstudent
Nov 19 '12 at 2:17
$begingroup$
OK. Use Cauchy's integral formula. There is only one pole to be picked up on the right hand side, which is at $z_0 = f^{-1}(w)$. So we only need to compute the residue. Near $z_0$, write $f(z)$ as $w + (z-z_0)^k h(z)$, where $h(z_0) neq 0$. Then $f'(z) = k(z-z_0)^{k-1} (h(z) + (z-z_0)h'(z)/k)$, where the thing inside the bracket does not vanish at $z_0$. Can you see what the residue at $z_0$ is now?
$endgroup$
– user27126
Nov 19 '12 at 6:53
add a comment |
$begingroup$
After some thought, it makes sense after applying the Cauchy Integral Formula to the inverse function and then making the substitution $ f^{-1}(s) = t $ where t is taken along the path given by $f^{-1}(s)$ where s is along $partial D(P,r)$.
The problem is being able to deform that curve into a circle which goes around $f^{-1}(w)$. This can be done if we can show the path only goes around $f^{-1}(w)$ once.
I still have no idea how the argument principle is involved
$endgroup$
After some thought, it makes sense after applying the Cauchy Integral Formula to the inverse function and then making the substitution $ f^{-1}(s) = t $ where t is taken along the path given by $f^{-1}(s)$ where s is along $partial D(P,r)$.
The problem is being able to deform that curve into a circle which goes around $f^{-1}(w)$. This can be done if we can show the path only goes around $f^{-1}(w)$ once.
I still have no idea how the argument principle is involved
edited Nov 15 '12 at 18:05
Martin Argerami
125k1181180
125k1181180
answered Nov 15 '12 at 17:41
MathstudentMathstudent
512
512
$begingroup$
It's just something very similar to the argument principle (in particular the proof), eg, see the answer. Or just look at the special case $w = 0$, and compare the statements and the proofs of this and argument principle.
$endgroup$
– user27126
Nov 15 '12 at 17:59
$begingroup$
I would but I still don't have a proof of this, can you direct me to one?
$endgroup$
– Mathstudent
Nov 15 '12 at 18:15
$begingroup$
What's wrong with the other answer?
$endgroup$
– user27126
Nov 15 '12 at 18:57
$begingroup$
The other answer is a hint, not a proof of the the above, which I am still confused on.
$endgroup$
– Mathstudent
Nov 19 '12 at 2:17
$begingroup$
OK. Use Cauchy's integral formula. There is only one pole to be picked up on the right hand side, which is at $z_0 = f^{-1}(w)$. So we only need to compute the residue. Near $z_0$, write $f(z)$ as $w + (z-z_0)^k h(z)$, where $h(z_0) neq 0$. Then $f'(z) = k(z-z_0)^{k-1} (h(z) + (z-z_0)h'(z)/k)$, where the thing inside the bracket does not vanish at $z_0$. Can you see what the residue at $z_0$ is now?
$endgroup$
– user27126
Nov 19 '12 at 6:53
add a comment |
$begingroup$
It's just something very similar to the argument principle (in particular the proof), eg, see the answer. Or just look at the special case $w = 0$, and compare the statements and the proofs of this and argument principle.
$endgroup$
– user27126
Nov 15 '12 at 17:59
$begingroup$
I would but I still don't have a proof of this, can you direct me to one?
$endgroup$
– Mathstudent
Nov 15 '12 at 18:15
$begingroup$
What's wrong with the other answer?
$endgroup$
– user27126
Nov 15 '12 at 18:57
$begingroup$
The other answer is a hint, not a proof of the the above, which I am still confused on.
$endgroup$
– Mathstudent
Nov 19 '12 at 2:17
$begingroup$
OK. Use Cauchy's integral formula. There is only one pole to be picked up on the right hand side, which is at $z_0 = f^{-1}(w)$. So we only need to compute the residue. Near $z_0$, write $f(z)$ as $w + (z-z_0)^k h(z)$, where $h(z_0) neq 0$. Then $f'(z) = k(z-z_0)^{k-1} (h(z) + (z-z_0)h'(z)/k)$, where the thing inside the bracket does not vanish at $z_0$. Can you see what the residue at $z_0$ is now?
$endgroup$
– user27126
Nov 19 '12 at 6:53
$begingroup$
It's just something very similar to the argument principle (in particular the proof), eg, see the answer. Or just look at the special case $w = 0$, and compare the statements and the proofs of this and argument principle.
$endgroup$
– user27126
Nov 15 '12 at 17:59
$begingroup$
It's just something very similar to the argument principle (in particular the proof), eg, see the answer. Or just look at the special case $w = 0$, and compare the statements and the proofs of this and argument principle.
$endgroup$
– user27126
Nov 15 '12 at 17:59
$begingroup$
I would but I still don't have a proof of this, can you direct me to one?
$endgroup$
– Mathstudent
Nov 15 '12 at 18:15
$begingroup$
I would but I still don't have a proof of this, can you direct me to one?
$endgroup$
– Mathstudent
Nov 15 '12 at 18:15
$begingroup$
What's wrong with the other answer?
$endgroup$
– user27126
Nov 15 '12 at 18:57
$begingroup$
What's wrong with the other answer?
$endgroup$
– user27126
Nov 15 '12 at 18:57
$begingroup$
The other answer is a hint, not a proof of the the above, which I am still confused on.
$endgroup$
– Mathstudent
Nov 19 '12 at 2:17
$begingroup$
The other answer is a hint, not a proof of the the above, which I am still confused on.
$endgroup$
– Mathstudent
Nov 19 '12 at 2:17
$begingroup$
OK. Use Cauchy's integral formula. There is only one pole to be picked up on the right hand side, which is at $z_0 = f^{-1}(w)$. So we only need to compute the residue. Near $z_0$, write $f(z)$ as $w + (z-z_0)^k h(z)$, where $h(z_0) neq 0$. Then $f'(z) = k(z-z_0)^{k-1} (h(z) + (z-z_0)h'(z)/k)$, where the thing inside the bracket does not vanish at $z_0$. Can you see what the residue at $z_0$ is now?
$endgroup$
– user27126
Nov 19 '12 at 6:53
$begingroup$
OK. Use Cauchy's integral formula. There is only one pole to be picked up on the right hand side, which is at $z_0 = f^{-1}(w)$. So we only need to compute the residue. Near $z_0$, write $f(z)$ as $w + (z-z_0)^k h(z)$, where $h(z_0) neq 0$. Then $f'(z) = k(z-z_0)^{k-1} (h(z) + (z-z_0)h'(z)/k)$, where the thing inside the bracket does not vanish at $z_0$. Can you see what the residue at $z_0$ is now?
$endgroup$
– user27126
Nov 19 '12 at 6:53
add a comment |
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$begingroup$
You should assume that $|f^{-1}(w)-P|<r.$
$endgroup$
– P..
Nov 12 '12 at 7:06