From 28407333ffceca9b99fae721c30e8ae146a863da Mon Sep 17 00:00:00 2001 From: Justin Gassner Date: Wed, 14 Feb 2024 07:24:38 +0100 Subject: Update --- .../one-complex-variable/power-series.md | 76 +++++++++++----------- 1 file changed, 37 insertions(+), 39 deletions(-) (limited to 'pages/complex-analysis/one-complex-variable/power-series.md') diff --git a/pages/complex-analysis/one-complex-variable/power-series.md b/pages/complex-analysis/one-complex-variable/power-series.md index 0147f31..31793ab 100644 --- a/pages/complex-analysis/one-complex-variable/power-series.md +++ b/pages/complex-analysis/one-complex-variable/power-series.md @@ -3,59 +3,57 @@ title: Power Series parent: One Complex Variable grand_parent: Complex Analysis nav_order: 1 -# cspell:words --- # {{ page.title }} -{: .definition-title } -> Definition ({{ page.title }}) -> -> Let $X$ be a complex Banach space. -> A *power series* (with values in $X$) is an infinite series of the form -> -> -> $$ -> \sum_{n=0}^{\infty} x_n (z - a)^n = x_0 + x_1 (z-a) + x_2 (z-a)^2 + \cdots, -> $$ -> -> where $x_n \in X$ is the *$n$th coefficient*, -> $z$ is a complex variable and -> $a$ is the *center* of the series. - -{: .lemma } -> Suppose the Banach space valued power series $\sum_{n=0}^{\infty} x_n (z - a)^n$ converges for $z = a + w$. -> Then it converges absolutely for all $z$ with $\abs{z-a} < \abs{w}$. +{% definition Power Series %} +Let $X$ be a complex Banach space. +A *power series* (with values in $X$) is an infinite series of the form + +$$ +\sum_{n=0}^{\infty} x_n (z - a)^n = x_0 + x_1 (z-a) + x_2 (z-a)^2 + \cdots, +$$ + +where $x_n \in X$ is the *$n$th coefficient*, +$z$ is a complex variable and +$a$ is the *center* of the series. +{% enddefinition %} + +{% lemma %} +Suppose the Banach space valued power series $\sum_{n=0}^{\infty} x_n (z - a)^n$ converges for $z = a + w$. +Then it converges absolutely for all $z$ with $\abs{z-a} < \abs{w}$. +{% endlemma %} {% proof %} TODO {% endproof %} -{: .theorem } -> Suppose $\sum_{n=0}^{\infty} x_n (z - a)^n$ is a Banach space valued power series. -> Then either -> -> - the series converges only for $z=a$ (formally $R=0$), or -> - there exists a number $0 the series converges absolutely whenever $\abs{z-a} < R$ -> and diverges whenever $\abs{z-a} > R$, or -> - the series converges absolutely for all $z \in \CC$ (formally $R=\infty$). -> -> The number $R \in [0,\infty]$ is called the *radius of convergence* of the power series. +{% theorem %} +Suppose $\sum_{n=0}^{\infty} x_n (z - a)^n$ is a Banach space valued power series. +Then either + +- the series converges only for $z=a$ (formally $R=0$), or +- there exists a number $0 R$, or +- the series converges absolutely for all $z \in \CC$ (formally $R=\infty$). + +The number $R \in [0,\infty]$ is called the *radius of convergence* of the power series. +{% endtheorem %} {% proof %} TODO {% endproof %} -{: .theorem-title } -> Cauchy–Hadamard Formula -> -> Let $\sum_{n=0}^{\infty} x_n (z - a)^n$ be a Banach space valued power series -> with radius of convergence $R$. Then -> -> $$ -> \frac{1}{R} = \limsup_{n \to \infty} \norm{x_n}^{1/n}. -> $$ +{% theorem * Cauchy–Hadamard Formula %} +Let $\sum_{n=0}^{\infty} x_n (z - a)^n$ be a Banach space valued power series +with radius of convergence $R$. Then + +$$ +\frac{1}{R} = \limsup_{n \to \infty} \norm{x_n}^{1/n}. +$$ +{% endtheorem %} {% proof %} TODO -- cgit v1.2.3-54-g00ecf