Rappel de cours
Séries entières d’une variable réelle ou complexe.
I-Séries entières d’une variable complexe
Définition 1. On appelle série entière toute série de fonctions de la forme
où
et
une suite de nombres complexes
.
![Rendered by QuickLaTeX.com \sum\limits_{n\in \mathbb N }a_nz^n](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-0af1bd3e5289abd1663b694fbdcaee76_l3.png)
![Rendered by QuickLaTeX.com z \in \mathbb C](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-bdbf2a81af51b456020854b19e920e9d_l3.png)
![Rendered by QuickLaTeX.com (a_n)_{n \in \mathbb N }](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-fbe210417c9f8f4e6abe59e8251cc073_l3.png)
Définition 2. :
est appelé rayon de convergence de la série
.
![Rendered by QuickLaTeX.com R = sup \left\{r \in \mathbb R \quad \vert \quad (a_n)_{n \in \mathbb N } ~~\text{soit bornée} \right\} \in \mathbb R^⁺\cup \{+\infty \}](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-c5d53caff7f4a5d5c75ee1c9545559f4_l3.png)
![Rendered by QuickLaTeX.com \sum\limits_{n\in \mathbb N }a_nz^n](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-0af1bd3e5289abd1663b694fbdcaee76_l3.png)
Proposition 1. On considère
une série entière de rayon de convergence
. Soit
un élément de
.
![Rendered by QuickLaTeX.com \sum\limits_{n\in \mathbb N}a_nz^n](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-8dcee0aeb68637ab391f48c4d3f13573_l3.png)
![Rendered by QuickLaTeX.com R](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-e4c39c3dd18ac9bc8a5877c397f8bd52_l3.png)
![Rendered by QuickLaTeX.com z](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-e7a454d36f22401803b6231837fb9361_l3.png)
![Rendered by QuickLaTeX.com \mathbb C](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-80dbc5f9e4f5cde90fdbe950988bc3f7_l3.png)
- Si
alors la série
est absolument convergente,
- si
alors la série
est divergente.
Détermination pratique du rayon de convergence.
Soit![Rendered by QuickLaTeX.com \sum\limits_{n\in \mathbb N }a_nz^n](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-0af1bd3e5289abd1663b694fbdcaee76_l3.png)
![Rendered by QuickLaTeX.com R](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-e4c39c3dd18ac9bc8a5877c397f8bd52_l3.png)
-
Règle de D’Alembert : Si
alors
Règle de de Cauchy Sialors
.
- Règle d’Hadamard: Si
alors
.
Propriétés de la somme d’une série entière
On désigne par![Rendered by QuickLaTeX.com D(0,R)=\{ z \in \mathbb C, ~ \vert z \vert < R \}](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-ac879294a004a0cea583bf84bc47a54c_l3.png)
![Rendered by QuickLaTeX.com \mathbb C](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-80dbc5f9e4f5cde90fdbe950988bc3f7_l3.png)
![Rendered by QuickLaTeX.com R](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-e4c39c3dd18ac9bc8a5877c397f8bd52_l3.png)
![Rendered by QuickLaTeX.com \overline{D(0,R)}=\{ z \in \mathbb C, ~ \vert z \vert \leq R \}](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-6514f404a314334f07117bb9cc9fff30_l3.png)
![Rendered by QuickLaTeX.com \mathbb C](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-80dbc5f9e4f5cde90fdbe950988bc3f7_l3.png)
![Rendered by QuickLaTeX.com R](https://spe-maths.fr/wp-content/ql-cache/quicklatex.com-e4c39c3dd18ac9bc8a5877c397f8bd52_l3.png)
Exercice 1
Corrigé
Exercice 2