From 28407333ffceca9b99fae721c30e8ae146a863da Mon Sep 17 00:00:00 2001 From: Justin Gassner Date: Wed, 14 Feb 2024 07:24:38 +0100 Subject: Update --- .../cauchys-integral-formula.md | 75 +++++++++------------- 1 file changed, 32 insertions(+), 43 deletions(-) (limited to 'pages/complex-analysis/one-complex-variable/cauchys-integral-formula.md') diff --git a/pages/complex-analysis/one-complex-variable/cauchys-integral-formula.md b/pages/complex-analysis/one-complex-variable/cauchys-integral-formula.md index ccdd0ea..3cf81f7 100644 --- a/pages/complex-analysis/one-complex-variable/cauchys-integral-formula.md +++ b/pages/complex-analysis/one-complex-variable/cauchys-integral-formula.md @@ -3,39 +3,33 @@ title: Cauchy's Integral Formula parent: One Complex Variable grand_parent: Complex Analysis nav_order: 3 -# cspell:words --- # {{ page.title }} -{: .theorem-title } -> {{ page.title }} -> -> Let $f : G \to \CC$ be a function holomorphic in an open subset $G \subset \CC$. -> Let $\gamma$ be a contour in $G$ such that the interior of $\gamma$ is contained in $G$. -> Then for any point $a$ in the interior of $\gamma$, -> -> $$ -> f(a) = \frac{1}{2 \pi i} \int_{\gamma} \frac{f(z)}{z-a} \, dz. -> $$ -> {: .katex-display .mb-0 } +{% theorem * Cauchy's Integral Formula %} +Let $f : G \to \CC$ be a function holomorphic in an open subset $G \subset \CC$. +Let $\gamma$ be a contour in $G$ such that the interior of $\gamma$ is contained in $G$. +Then for any point $a$ in the interior of $\gamma$, + +$$ +f(a) = \frac{1}{2 \pi i} \int_{\gamma} \frac{f(z)}{z-a} \, dz. +$$ +{% endtheorem %} {% proof %} {% endproof %} -{: .theorem-title } -> {{ page.title }} (Generalization) -> {: #cauchys-integral-formula-generalized } -> -> Let $f : G \to \CC$ be a function holomorphic in an open subset $G \subset \CC$. -> Then the $n$th derivative $f^{(n)}$ exists for every $n \in \NN$. -> If $\gamma$ is a contour in $G$ such that the interior of $\gamma$ is contained in $G$, -> then for any point $a$ in the interior of $\gamma$, -> -> $$ -> f^{(n)} (a) = \frac{n!}{2 \pi i} \int_{\gamma} \frac{f(z)}{(z-a)^{n+1}} \, dz. -> $$ -> {: .katex-display .mb-0 } +{% theorem * Cauchy's Integral Formula (Generalization) %} +Let $f : G \to \CC$ be a function holomorphic in an open subset $G \subset \CC$. +Then the $n$th derivative $f^{(n)}$ exists for every $n \in \NN$. +If $\gamma$ is a contour in $G$ such that the interior of $\gamma$ is contained in $G$, +then for any point $a$ in the interior of $\gamma$, + +$$ +f^{(n)} (a) = \frac{n!}{2 \pi i} \int_{\gamma} \frac{f(z)}{(z-a)^{n+1}} \, dz. +$$ +{% endtheorem %} {% proof %} {% endproof %} @@ -50,20 +44,17 @@ and is often used to compute the integral. ## Many Consequences -{: .theorem-title } -> Cauchy's Estimate -> {: #cauchys-estimate } -> -> Let $f$ be holomorphic on an open set containing the disc with center $a$ and radius $r>0$. -> Then -> -> $$ -> \norm{f^{(n)}(a)} \le \frac{n!}{r^n} \sup_{\abs{z-a} = r} \norm{f(z)} \qquad \forall n \in \NN. -> $$ -> {: .katex-display .mb-0 } +{% theorem * Cauchy's Estimate %} +Let $f$ be holomorphic on an open set containing the disc with center $a$ and radius $r>0$. +Then + +$$ +\norm{f^{(n)}(a)} \le \frac{n!}{r^n} \sup_{\abs{z-a} = r} \norm{f(z)} \qquad \forall n \in \NN. +$$ +{% endtheorem %} {% proof %} -From [{{ page.title }}](#cauchys-integral-formula-generalized) +From [{{ page.title }}](#cauchy-s-integral-formula-generalization) for the circular contour around $a$ with radius $r$ we obtain $$ @@ -82,16 +73,14 @@ and the right hand side of the inequality reduces to the desired expression. Recall that an *entire* function is a holomorphic function that is defined everywhere in the complex plane. -{: .theorem-title } -> Liouville's Theorem -> {: #liouvilles-theorem } -> -> Every bounded entire function is constant. +{% theorem * Liouville's Theorem %} +Every bounded entire function is constant. +{% endtheorem %} {% proof %} Consider an entire function $f$ and assume that $\norm{f(z)} \le M$ for all $z \in \CC$ and some $M > 0$. Since $f$ is holomorphic on the whole plane, we may make -[Cauchy's Estimate](#cauchys-estimate) +[Cauchy's Estimate](#cauchy-s-estimate) for all disks centered at any point $a \in \CC$ and with any radius $r>0$. For the first derivative, we have $\norm{f'(a)} \le M/r$, which tends to $0$ for $r \to \infty$. Hence $f' = 0$ in the whole plane. This -- cgit v1.2.3-70-g09d2