Jensen's Formula
Theorem
Let $S$ be an open subset of the complex plane containing the closed disk:
- $D_r = \set {z \in \C : \cmod z \le r}$
of radius $r$ about $0$.
Let $f: S \to \C$ be holomorphic on $S$.
Let $f$ have no zeroes on the circle $\cmod z = r$.
Let $\map f 0 \ne 0$.
Let $\rho_1, \ldots, \rho_n$ be the zeroes of $f$ in $D_r$, counted with multiplicity.
Then:
- $(1): \quad \ds \frac 1 {2 \pi} \int_0^{2 \pi} \ln \cmod {\map f {r e^{i \theta} } } \rd \theta = \ln \cmod {\map f 0} + \sum_{k \mathop = 1}^n \paren {\ln r - \ln \size {\rho_k} }$
Proof 1
Write:
- $\map f z = \paren {z - \rho_1} \dotsm \paren {z - \rho_n} \map g z$
so $\map g z \ne 0$ for $z \in D_r$.
It is sufficient to check the equality for each factor of $f$ in this expansion.
First let:
- $\map h z = z - \rho_k$
for some $k \in \set {1, \ldots, n}$.
Making use of the substitution $u = r e^{i \theta} - \rho_k$ we find that:
- $\ds \frac 1 {2 \pi} \int_0^{2 \pi} \ln \size {\map h {r e^{i \theta} } } \rd \theta = \frac 1 {2 \pi i} \int_\gamma \frac {\ln \size u} {u + \rho_k} \rd u$
where $\gamma$ is a circle of radius $r$ centred at $-\rho_k$, traversed anticlockwise.
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On this circle, $\ln \size u = \ln r$ is constant, and we have that:
- $\ds \int_\gamma \frac 1 {u + \rho_k} \rd u = \int_{\size z \mathop = r} \frac {\d u} u = 2 \pi i$
Therefore the left hand side of $(1)$ is $\ln r$ as required.
To show equality for $\map g z$, first observe that by Cauchy's Residue Theorem:
- $\ds \int_{\size z = r} \frac {\ln \map g z} z \rd z = 2 \pi i \ln \map g 0$
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Therefore substituting $z = r e^{i \theta}$ we have
- $\ds 2 \pi i \ln \map g 0 = i \int_0^{2 \pi} \ln \map g {r e^{i \theta} } \rd \theta$
Comparing the imaginary parts of this equality we see that:
- $\ds \frac 1 {2 \pi} \int_0^{2 \pi} \ln \size {\map g {r e^{i \theta} } } \rd \theta = \ln \size {\map g 0}$
as required.
$\blacksquare$
Proof 2
Write
- $\map f z = \dfrac {r^2 - z \overline {\rho_1} } {r \paren {z - \rho_1} } \cdots \dfrac {r^2 - z \overline {\rho_n} } {r \paren {z - \rho_n} } \map g z$
so $\map g z \ne 0$ for $z \in D_r$.
This article, or a section of it, needs explaining. In particular: Why can you write like this? Looks wrong, because the zeros do not correspond. For example, insert $z=\rho_1$ in both sides. You can help $\mathsf{Pr} \infty \mathsf{fWiki}$ by explaining it. To discuss this page in more detail, feel free to use the talk page. When this work has been completed, you may remove this instance of {{Explain}} from the code. |
It is sufficient to check equality for each factor of $f$ in this expansion.
When $\cmod z = r$, we have:
- $\dfrac 1 z = \dfrac {\overline z} {r^2}$
and:
- $\cmod {\dfrac z r} = 1$
where $\overline z$ denotes the complex conjugate of $z$.
So:
\(\ds \cmod {\frac {r^2 - z \overline {\rho_k} } {r \paren {z - \rho_k} } }\) | \(=\) | \(\ds \frac {\cmod {r^2 - \overline z \rho_k} } {\cmod {r \paren {z - \rho_k} } }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac {\cmod {1 - \overline z \rho_k / r^2} } {\cmod {z / r - \rho_k / r} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac {\cmod {1 - \overline z \rho_k / r^2} } {\cmod {1 - \rho_k / z} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac {\cmod {1 - \overline z \rho_k / r^2} } {\cmod {1 - \overline z \rho_k / r^2} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds 1\) |
so the left hand side is $0$.
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Moreover:
- $\\ln \cmod {\dfrac {r^2 - 0 \overline {\rho_k} } {r \paren {0 - \rho_k} } } = \ln \cmod {\rho_k} - \ln r$
so the right hand side is $0$.
Therefore, the formula holds for $\dfrac {r^2 - z \overline {\rho_k} } {r \paren {z - \rho_k} }$.
Since:
- $r^2 - z \overline {\rho_i} = 0 \implies \cmod z = r^2 / \cmod {\overline {\rho_i} } > r$
it follows that $\map g z$ is holomorphic without zeroes on $D_r$.
So the right hand side is $\ln \size {\map g 0}$.
On the other hand, by the mean value property:
- $\ds \dfrac 1 {2 \pi} \int_0^{2 \pi} \ln \size {\map g {r e^{i \theta} } } \rd \theta = \ln \map g 0$
as required.
$\blacksquare$
Proof 3
Without loss of generality, assume that $r = 0$.
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Write:
- $\map f z = \map {B_{\rho_1} } z \dotsm \map {B_{\rho_n} } z \map g z$
where $\map {B_\rho} z := \dfrac {z - \rho} {1 - z \overline \rho}$ is the Blaschke factor at $\rho$.
So:
- $\map g z$ is holomorphic and nonzero in $D_r$
and:
- each $\map {B_{\rho_i} } z$ is holomorphic and has a simple zero $\rho_i$ inside $D_r$.
It is sufficient to check the equality for each factor of $f$ in this expansion.
First let:
- $\map h z = \map {B_{\rho_k} } z = \dfrac {z - \rho_k} {1 - z \overline {\rho_k} }$
for some $k \in \set {1, \ldots, n}$.
Note that:
- $\size {\map h z} \equiv 1$ for $\size z = 1$
so the integral is $0$.
Also:
- $\map {B_{\rho_k} } 0 = \rho_k$
so the equality holds in this case.
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To show equality for $\map g z$, first observe that by the Cauchy's Residue Theorem:
- $\ds \int_{\size z \mathop = r} \frac {\ln \map g z} z \rd z = 2 \pi i \ln \map g 0$
This article, or a section of it, needs explaining. In particular: Is $\ln \map g z$ well-defined? You can help $\mathsf{Pr} \infty \mathsf{fWiki}$ by explaining it. To discuss this page in more detail, feel free to use the talk page. When this work has been completed, you may remove this instance of {{Explain}} from the code. |
Therefore substituting $z = r e^{i \theta}$ we have:
- $\ds 2 \pi i \ln \map g 0 = i \int_0^{2 \pi} \ln \map g {r e^{i \theta} } \rd \theta$
Comparing the imaginary parts of this equality we see that:
- $\ds \frac 1 {2 \pi} \int_0^{2 \pi} \ln \size {\map g {r e^{i \theta} } } \rd \theta = \ln \size {\map g 0}$
as required.
$\blacksquare$
Source of Name
This entry was named for Johan Jensen.